Enhanced Activity for Hydrogen Evolution Reaction over CoFe

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Enhanced activity for Hydrogen Evolution Reaction over CoFe Catalysts by Alloying with small amount of Pt Jitang Chen, Yang Yang, Jianwei Su, Peng Jiang, Guoliang Xia, and Qianwang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12065 • Publication Date (Web): 12 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

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ACS Applied Materials & Interfaces

Enhanced activity for Hydrogen Evolution Reaction over CoFe Catalysts by Alloying with small amount of Pt Jitang Chen1,3, Yang Yang1, Jianwei Su1, Peng Jiang1, Guoliang Xia1, and Qianwang Chen1,2* 1

Hefei National Laboratory for Physical Science at Microscale, Department of Materials Science &

Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei 230026, China 2

High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences,

Hefei 230031, China. 3

School of Chemistry and Materials Engineering, Fuyang Normal University, Fuyang 236041, China.

Keywords: metal-organic framewok,

trimetallic, alloy, hydrogen generation, synergistic

effect

Abstract The hydrogen evolution reaction (HER) highly relied on Pt electrocatalysts, with high activity and stability. In the past a few years, a host of efforts have been made in the development of novel platinum nano-structures with a low amount of Pt because the scarcity and high price of Pt hinders its practical applications. Here, we report the preparation of PtCoFe@CN electrocatalysts with remarkably reduced Pt loading amount of 4.60% by annealing Pt-doped metal-organic frameworks (MOFs). The electrocatalyst demonstrated an outstanding performance with only 45 mV overpotential to achieve the 10 mA cm-2 current density, which is quite close to that of the commercial 20% Pt/C catalyst. The enhanced catalytic capability is originated from the modification of the electronic structures of CoFe by alloying with Pt. The results indicate that robust and super-stable alloy electrocatalysts which contain a very small amount of noble metal could be prepared by annealing noble metal doped MOFs.

1. Introduction Electrocatalysts, a crucial route for hydrogen evolution reactions (HER), have attracted tremendous attention for decades.

An ideal electrocatalyst for HER should be highly active,

ACS Paragon Plus Environment


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long-term stable and inexpensive for large-scale application. 1-2 Up to now, Pt is still considered as a state-of-the-art HER electrocatalyst due to its almost zero overpotential, relatively low Tafel slope and superior stability.

2-6

Unfortunately, the abundance of Pt is 0.005 ppm by weight within

the crust of the Earth and the corresponding cost is ca. $997 per ounce, which significantly hinders its widespread technological use. 7-9 Consequently, plenty of research efforts have been committed to developing inexpensive catalysts with extraordinary property and outstanding durability.

3,

10-12

Owing to the high abundance and low cost, transition metals had been explored as promising

candidate HER electrocatalysts.13-21 When transition metal nanoparticles (e.g., Fe, Co and Ni) were embedded in carbon (graphene, carbon nanotubes and etc.) especially heteroatoms-doped carbon they displayed positive electrocatalytic property in acidic or basic solution.14, 21-22 For example, Bao and his co-workers demonstrated that carbon-encapsulated alloy (FeCo and CoNi alloy) nanoparticles (NPs) bring out superior HER performance in contrast to their single metallic components. 13, 15 The density function theory (DFT) calculation results certified that the working function of the carbon shell could be modified by the intimate connection between the carbon shell and metallic core, and therefore delivering them with particularly high catalytic activities. Our group took a simple process to synthesize a transition metal alloy and nitrogen doped graphene hybrid material as effective and steady electrocatalyst by one-step annealing of MOFs precursor nanoparticles.

20

Despite some remarkable progress has been made in recent decades,

the HER catalytic activity is still far from satisfactory because of their high overpotential and low current density. 3, 15-16, 20-21 Another alternative method is to decrease the usage of precious metals in catalysts by incorporating first row transition metals (for example, Fe, Co and Ni) into noble metals to generate alloys.

10, 23-25

Nørskov and his coworkers introduced a screening procedure to evaluate

surface alloy of bismuth and platinum as the most active catalyst for HER and they got some positive experimental results. 23 Besides, previous studies have proved that the electronic structure, the lattice and bond length of noble metals can be tuned by alloying with non-noble metals so as to change adsorption energies toward optimal catalytic activity.

20, 24-25

However, the main

component of these electrocatalysts is the precious metals; so the challenge still remains due to the high cost of precious metals.

5, 26

Thus, a feasible option that could minimize the loading amount

of noble metal while retaining the electrocatalytic activity for HER is attractive. 27


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It is found that alloying is an effective method to enhance catalytic activity through adjusting the electronic structures of metals. 5, 23-26 It is assumed that ternary metallic alloys would provide more feasibility to further optimize electrocatalytic activity by increasing the freedom degrees of alloys more than binary alloys. However, few studies have focused on the graphene wrapped ternary alloys; especially those elaborated tuning of electronic structure and systemic investigation of the relationship between metal composition and HER activity. To compare and gain insights into the performance among different catalysts, it is of great significance to synthesize diverse electrocatalysts with semblable morphologies from a single precursor. 28 The competent and stable HER catalysts can be generated by calcining precious metal-doped MOFs precursors. 29 In the present work, we report the preparation of nitrogen-doped carbon encapsulated ternary PtCoFe alloy (PtCoFe@CN) by annealing MOFs precursor under N2 atmosphere. Electrochemical measurements showed that the as-prepared ternary alloy catalyst illustrates excellent hydrogen evolution reaction activity and long-term stability better than binary alloy in acidic solutions, matching with the value of single-atom catalysts2, 6, 30 and commercial Pt/C. DFT calculations indicate that incorporating Pt into transition metals could drive the modification of the electronic structures so as to the remarkable catalytic activity.

2. Results and discussions To prepare the alloy nanoparticle@nitrogen-doped carbon (PtCoFe@CN), we used a two-step approach: (a) sepia powder precursor was firstly prepared through a hydrothermal process via a modified method described in our previous work, 31 (b) the sepia powder was annealed in nitrogen atmosphere at the setting temperature and the black powder was the final product. The procedure was shown as scheme 1 and the detailed information was provided in the Supporting Information.



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Scheme 1 The synthetic illustration of graphene layers encapsulated PtCoFe ternary alloys. The uniform cubic of Pt-doped Co3[Fe(CN)6]2 with a particle size of 150 nm (Figure S 1 A, B) was constructed by a simple hydrothermal synthesis procedure. During the annealing process, cubes were collapsed into nanospheres and aggregated together (Figure S 1 C, D and Figure S 2 A, B). The transmission electron microscopy (TEM) images indicated that the hybrid PtCoFe@CN nanospheres are consisted of metal nanoparticles (NPs) that are completely encapsulated by carbon shells (Figure S 1 D and S 2). As shown in Figure S 3, the morphology of Pt-doped Co3[Co (CN)6]2 was nanosphere with a smooth surface (Figure S 3 A), yet relatively rough surface for the final product (Figure S 3 B). The TEM image of PtCo@CN reveals that there are many small encapsulated particles in the spherical product (Figure S 3 C).

Figure 1 The X-ray diffraction patterns of PtCoFe@CN (A); HRTEM image and the lattice spacing of PtCoFe alloy (B). PXRD diffraction patterns of precursor and annealed product were shown in Figure S 4 and Figure 1 A. The diffraction peaks of the precursor (Figure S 4) were good in line with the values of cobalt hexacyanoferrate (JPCDS: 75-0039).

31

Wide-angle X-ray diffraction (WAXD) of the final

product reveals two obvious peaks, which index the existence of (110) and (200) lattice plane of cubic FeCo alloy (JCPDS 49-1567),

13, 20

respectively. The diffraction peaks were sharp and


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intense, indicating their highly crystalline nature. It should be pointed out that the peak

positions are slightly shifted to lower angles, relative to the undoped FeCo alloy (Figure S 5), implying the lattice distortion of the cubic structure and revealing the

incorporation of Pt into alloy lattice. 32-34 Further High Resolution Transmission Electron Microscopy (HRTEM) analysis confirms the composite structure of the NPs. The lattice spacing was measured as 0.201 nm, which is matched with cubic FeCo (110) and indicated the forming of alloy phase (Figure 1 B). The essence of the doping in the hybrid can be disclosed by the elemental mapping images of C, N, Pt, Co and Fe (Figure 2 B−F). As expected, Pt, Co and Fe atoms are distributed homogeneously over the alloy core. Line-scan EDS analysis across a nanoparticle was implemented (Figure S 6), which demonstrates the trimetallic alloy was generated. The contents of Pt, Co and Fe in the alloy were detected to be 4.6, 55.4 and 40 wt% by ICP analysis.

Figure 2 TEM image of PtCoFe@CN (A); elemental mappings of C (B); N (C); Pt (D); Co (E); Fe (F). The Raman spectroscopy analysis is a powerful approach to obtain the characters of carbon, especially for graphitic carbon materials. 35-36 The Raman spectroscopy of the sample was carried and the results were shown as Figure S 7. The graphitic of carbon shell was indicated by the existence of the clearly D band and G band which can be seen at 1348 and 1578 cm-1. Moreover, the high ID/IG ratio of the sample suggesting abundant defects was in the carbon shell.

20, 35-36

Another thing should be noticed is that there is a weak 2D band was observed around 2700 cm-1, which is the characteristic peak of graphene with several layers.

20-21, 29, 32-33

The defect-rich and

graphene-like structure of the shell was confirmed by X-ray photoelectron spectroscopy analysis,



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which will be further discussed below. The surface composition of the hybrid nanoparticle was determined by XPS. As displayed in Figure S 8 A, two distinct metallic Pt peaks were arose at 71.9 and 75.1 eV, which correspond well to Pt4 f7/2 and Pt4 f5/2, respectively. Strikingly, the 4f peaks were shifted from 71.4 and 74.5 eV (pure Pt) to 71.9 and 75.1 eV (PtCoFe@CN) upon alloy formation with Co and Fe, which are in agreement with the previous reports.

37-38

Meanwhile, the upward shift of the core level of the

alloy is owing to the charge transfer from transition metal atoms to noble metal atoms.

29, 39

The

valence state of Co was confirmed by XPS (Figure S 8 B), the two peaks can be deconvoluted into six subpeaks, and which were corresponding with Co 2p and Co (II), separately.29, 39 Similary, from the spectrum of Fe 2p (Figure S 8 C), four peaks at 707, 720.4, 712 and 725.5 eV indicated the existence of the 2p orbitals of Fe and Fe (II) species, respectively.40-42 The shifts of binding energies for metal atoms implied that there exist interactions between the metal atoms and N atoms.39 The N 1s spectrum (Figure S 8 D) can be divided into several peaks at 398.3, 399.1, 400.2 and 401.4 eV, which are the typical binding energies of pyridinic N, pyrrolic N, and graphitic N, respectively. The interactions of N with metal atoms caused the energy shift and which was verified by binding energies at 399.1 and 400.2 eV. 20, 43

Figure 3 Polarization curves (A) and Tafel plots of samples (B); Polarization curves of PtCoFe@CN 1st and 10000th cycles (C); Amperometric i–t curves of PtCoFe@CN (D). To decode the electrocatalytic HER activities of the alloy catalyst, linear sweep voltammetry (LSV) were performed (SI) in N2-saturated 0.5 M H2SO4. As a reference, LSV curves of


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CoFe@CN, PtCo@CN and commercial 20% Pt/C were also be recorded in homologous solution. Polarization curve is an essential certification to estimate the hydrogen generation performance of electrocatalyst. As displayed in figure 3 A, CoFe@NC illustrates the lowest activity with the highest onsetpotential and Pt/C is the most active one with the smallest onset potential. The overpotential at 10 mA cm-2 (the current density expected for a 12.3% efficient solar water-splitting device) is usually considered as an important index of electrocatalyst.44 To achieve a 10 mA cm-2 HER current density, the trimetallic alloy catalyst required a low overpotential of 45 mV, which is less than that of bimetallic alloy catalysts CoFe@NC (260 mV) and PtCo@CN (94 mV). The performance of ternary catalyst is comparable with recent reports of Pt hybrid catalysts, 9, 45-46

single-atom catalysts 2, 6, 30 (Table S1). The contrast of polarization curves between 5% Pt/C

and PtCo@CN were presented in Figure S 9, the added potential of 5% Pt/C is obviously larger than PtCo@CN to approach equal current density. The result proves that the electrocatalytic performance could be obtained a significant increase by alloying effect. The sensitivity of electronic current response to driving potential is defined as the Tafel slope, which affords information concerning the rate determining steps. As a result, Tafel curve is commonly used for analyzing kinetic of the HER and other electrochemical reactions.

47

As

described in previous works, there are three major steps for hydrogen generation procedure in acidic solution, and the rate determining step (RDS) of a catalyst can disclosed by its Tafel slope.21, 47 Tafel plots derived from the polarization curves of the five samples were shown in figure 3 B. The value of Tafel slope for CoFe@CN, PtCo@CN, PtCo@CN and Pt/C catalyst is 110 mV dec−1, 46mV dec−1, 32 mV dec−1, and 30 mV dec−1, separately. Therefore, a logical verdict could be drawn from the value of PtCoFe@CN and Pt/C that the RDS of the two catalysts is similar, and the HER take place by the Volmer–Heyrovsky mechanism.21, 47-48 The small Tafel slope of the PtCoFe@CN represents its superior catalytic activity, because of the dramatic increase of HER velocity while overpotential increasing. Generally, electrochemically active surface area (ECSA) is regarded as an indicative reference point to estimate active sites of a HER catalyst, and the value of ECSA is relative to electrochemical double-layer capacitance (Cdl). The Cdl of CoFe@CN, PtCo@CN and PtCoFe@CN were measured by voltammetry, and the consequence was exhibited in Figure S 10. The rank of Cdl is following as CoFe@CN

Enhanced Activity for Hydrogen Evolution Reaction over CoFe

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