Research Article www.acsami.org
Metal−Organic Coordination Networks: Prussian Blue and Its Synergy with Pt Nanoparticles to Enhance Oxygen Reduction Kinetics Lei Du,†,‡,§ Chunyu Du,*,† Guangyu Chen,† Fanpeng Kong,† Geping Yin,*,‡ and Yong Wang§ †
Key Laboratory of Materials for New Energy Conversion and Storage, Ministry of Industry and Information Technology, Harbin Institute of Technology, Harbin 150001, China ‡ State Key Laboratory of Urban Water Resource and Environment, School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150090, China) § The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99164-6515, United States S Supporting Information *
ABSTRACT: Oxygen reduction reaction (ORR) is the cornerstone in the electrochemical energy conversion devices such as fuel cells and metal−air batteries. It remains a great challenge to develop the ORR electrocatalysts with fast kinetics and high durability. Herein, we report the synthesis of a novel metal−organic coordination networks material, prussian blue crystalline nanograins mosaicked within amorphous membrane (PB CNG-M-AM). Such unique PB CNG-M-AM is designed to enhance the electrocatalysis of Pt toward the ORR by the electrostatic self-assembly. Thus, obtained Pt-PB/C catalysts form numerous Pt-PB-gas threephase boundaries and present rather high intrinsic activity, four-electron selectivity and superior stability. Moreover, a completely new synergetic mechanism between PB and Pt is discovered, which delicately alters the ORR route and significantly enhances the ORR kinetics. This work provides not only a new strategy and mechanism for developing highly efficient ORR electrocatalysts, but also an alternative way to utilize metal−organic coordination networks materials. KEYWORDS: fuel cells, metal−organic coordination materials, oxygen reduction, prussian blue, platinum, synergy
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INTRODUCTION
significant progresses, it is still in urgent need to develop more efficient ORR electrocatalysts. Recently, metal−organic coordination network (MOCN) materials, among which metal−organic frameworks are the most representative, have become an intense research topic with potential applications in surface patterning,17 gas capture/ storage,18−20 catalysis,21 and molecular electronics.22 For electrocatalytic applications, MOCN materials are limited by their low electrical conductivity,23 so that only a few investigations on the pyrolized MOCN materials have been reported for ORR.24−27 However, the high-temperature pyrolysis may destroy the metal−organic coordination networks and essentially lead to the composites of metal/metal oxides and carbon. It is of significance to explore the electrocatalytic applications of MOCN materials with welldefined metal−organic structures. As a series of MOCN materials, prussian blue (PB) analogues possess a cubic structure formulated as M[M′(CN)6]
Oxygen reduction reaction (ORR) has been attracting tremendous attention because of its technological significance in many electrochemical energy conversion systems,1−5 such as polymer electrolyte fuel cells and metal−air batteries, which promise a low-toxic, high-efficient, eco-friendly and sustainable solution to the mounting threats from environmental destruction and energy shortage.6 At present, platinum (Pt) is commonly used as the ORR electrocatalyst. However, the use of Pt is significantly restricted because of its scarce resources and high price. Moreover, the small amount of oxidizing H2O2 generated via two-electron transfer process on Pt is rather detrimental to components of the energy conversion systems, especially the polymer electrolytes in fuel cells.7−9 Additionally, Pt catalyst usually undergoes severe degradation during the long-term operation through Ostwald ripening, agglomeration, dissolution and/or support corrosion.10 To address these issues, current efforts mainly focus on Pt-based alloys,11,12 preferential facets,13−15 and core−shell (monolayer) structures,16 which enhance the ORR properties by adjusting electronic and/or geometric structures.11−16 Despite these © 2016 American Chemical Society
Received: March 2, 2016 Accepted: June 2, 2016 Published: June 2, 2016 15250
DOI: 10.1021/acsami.6b02630 ACS Appl. Mater. Interfaces 2016, 8, 15250−15257
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic structure of PB CNG-M-AM sample. (b) TEM image and (c) corresponding SAED image of PB CNG-M-AM sample, presenting special misty and amorphous morphology, rather than the common nanocrystals. (d) Representative HRTEM image of small crystalline nanograins in PB CNG-M-AM sample. (e) XRD pattern of PB CNG-M-AM sample.
containing H2O2 is ultrasonically mixed with K4FeII(CN)6 solution. Immediately, the reaction solution becomes blue (Figure S2). After the reaction, the prominent peak at 2000− 2100 cm−1 in the infrared spectra of reaction product, which is assigned to the − CN− bond, shifts positively by ca. 32 cm−1 relative to K4[FeII(CN)6] (Figure S3), indicating the bonding of Fe3+ with [FeII(CN)6]4−.34 These observations clearly indicate the formation of PB. Before further characterizations, the obtained PB product was washed with 0.5 M H2SO4, water and ethonal to remove the possible Fe-containing and other impurities. Figure 1b gives transmission electron microscopy (TEM) image of thus synthesized PB CNG-M-AM sample, which presents misty and amorphous morphology, rather than large PB nanocrystals (Figure S1). The amorphous nature of PB CNG-M-AM sample is confirmed by the broadened electron diffraction circles in its selected area electron diffraction (SAED) pattern (Figure 1c). The detailed structure is revealed by high-rosolution TEM (HRTEM) image (Figure 1d). This PB CNG-M-AM sample appears to be amorphous, in agreement with the TEM image. However, careful observation reveals that very small crystalline nanograins with stable lattices (indicated by arrows), which are assigned to the crystalline PB, are mosaicked within the amorphous substrate. The detailed lattice fringes of these nanograins are provided in Figure S4, which shows various exposed facets such as PB (400), (420), and (440) facets (prussian blue JCPDS# 52−1907). The X-ray diffraction (XRD) pattern (Figure 1e) shows diffraction peaks that agree well with the standard pattern of PB, further confirming the presence of crystalline PB. Moreover, the relatively broadened XRD peaks verify its small crystalline size. All the results clearly illustrate the unique CNG-M-AM structure of PB. The key to the formation of CNG-M-AM structure is the delicate employment of H2O2 during the synthesis. It has been reported that Fe 3+ ions can facilely catalyze the H2O 2 decomposition, forming amorphous intermediates, such as Fe(OH)3, and oxygen product.35 We believe that this catalytic reaction consumes most of Fe3+ so that after [FeII(CN)6]4− is
(M and M′ represent metal elements), in which the octahedral [M′(CN)6]n− complexes coordinate with nitrogen-bound Mn+ ions.28 Particularly, PB consists of alternating FeII and FeIII ions bridged by organic cyanide ions in an FeII−CN−FeIII style (Figure 1a). Herein, we have designed and synthesized novel PB crystalline nanograins mosaicked within amorphous membrane (CNG-M-AM), which are employed as a new cocatalyst to promote the ORR properties of Pt. PB is recognized as one of the most advantageous catalysts for H2O2 decomposition,29,30 which can be expected to remarkably reduce the H 2 O 2 production during the ORR. More importantly, a new synergetic mechanism between PB and Pt is discovered, which is completely distinctive from the conventional electronic and geometric effects. This mechanism can delicately alter the ORR route and significantly enhance the ORR kinetics. Further, PB CNG-M-AM can prevent Pt nanoparticles (NPs) from migrating, dissolving, or detaching from supports, maintaining the activity over long time. Consequently, such carbon supported Pt and PB catalysts (denoted as Pt-PB/C) exhibit an unprecedented combination of excellent activity, four-electron selectivity and stability toward the ORR, especially for practical working potentials. Our findings not only propose a new synergetic mechanism of enhancing the ORR catalytic properties of Pt, but also provide a unique stragey to effectively utilize the MOCN mateirals in electrochemical fields.
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RESULTS AND DISCUSSION Synthesis and Characterization of PB CNG-M-AM and Pt-PB/C Catalyst. In previous reports, for applications, the MOCN PB mateiral is generally synthesized by mixing the solutions of Fe 2+ and [Fe III (CN) 6 ] 3− or Fe 3+ and [FeII(CN)6]4−.28,31−33 These synthetic procedures always lead to large PB nanocrystals (Figure S1), which are highly incompatible with Pt NPs and carbon supports in terms of physical contact and electron transfer for electrochemical applications. To address this challenge, we delicately introduce H2O2 into the synthetic process of PB. Briefly, FeCl3 solution 15251
DOI: 10.1021/acsami.6b02630 ACS Appl. Mater. Interfaces 2016, 8, 15250−15257
Research Article
ACS Applied Materials & Interfaces being added, the small amount of Fe3+ ions remained in the solution react rapidly with [FeII(CN)6]4− to form crystalline PB nanograins. As the Fe3+ ions are being depleted, the amorphous intermediate Fe(OH)3 reversibly releases Fe3+ ions slowly,35 which continuously react with [Fe II(CN)6]4− to form amorphous PB membrane. Simultaneously, O2 bubbles generated during the H2O2 decomposition disturb the reaction process, which is also beneficial to forming amorphous PB. By these unique processes, the novel PB CNG-M-AM structure is obtained, rather than forming large MOCN PB nanocrystals. The unique PB CNG-M-AM structure is used to enhance the electrocatalytic properties of Pt-based catalysts, which is achieved by the electrostatic self-assembly of positively charged Pt NPs protected by poly(diallyldimethylammonium chloride) PDDA36 and negatively charged PB37 on Vulcan XC-72R carbon black supports (Figure S5). By this means, Pt NPs and PB CNG-M-AM are uniformly loaded on the carbon support, and numerous Pt-PB-gas three phase boundaries are formed. In order to exclude the possible effects from other Fe-contaning species, the catalysts are also washed with 0.5 M H2SO4 to remove the impurities before further operation. The Pt loading of the targeted Pt-PB(n:m)/C catalysts is controlled at 20 wt % (n:m refers to the mass ratio of Pt to PB). The specific composition of Pt and PB in the catalysts is measured by inductively coupled plasma (ICP) and listed in Table S1, which is consistent with the nominal value. Figure 2a shows the representative TEM image of typical PtPB(4:1)/C catalyst. Pt NPs with average particle size of ∼2.3
Electrocatalytic Activity. Electrocatalytic performance of the Pt-PB/C catalysts and reference catalysts, including commercial Pt/C from Johnson-Matthey and Pt/C-PDDA synthesized by the similar method, toward the ORR was evaluated by carrying out the rotating ring-disk electrode (RRDE) experiments in an O2-saturated 0.1 M HClO4 solution. The potential of ring electrode was fixed at 1.2 V (vs. reversible hydrogen electrode, RHE, the same hereinafter) to detect the H2O2 produced during the ORR. Figure 3a gives H2O2 yield on the ring electrode as a function of the disk electrode potential for different ratios of Pt to PB. The H2O2 yield for all Pt-PB/C samples are relatively small compared with Pt/C and Pt/C-PDDA catalysts. As the PB content increases, however, the oxidation current declines significantly, clearly confirming the positive effect of PB on reducing H2O2 formation, because PB is one of the most advantageous catalysts for the H2O2 decomposition. Figure 3b shows the linear sweep voltammetry (LSV) polarization curves on the disk electrode at a scanning rate of 5 mV s−1 and rotation speed of 1600 rpm. The ORR kinetics is dramatically accelerated on some Pt-PB/C catalysts, relative to Pt/C and Pt/C-PDDA catalysts, illustrating the significant cocatalytic capability of the PB CNG-M-AM. Among them, the Pt-PB(4:1)/C catalyst exhibits the highest ORR activity. Such optimal ratio of Pt to PB arises probably from a balance between the electronic conductivity and cocatalytic capability of PB CNG-M-AM, which is verified by the high ohmic resistance as the PB CNGM-AM content increases (Figure S8). From the TEM images (Figure S9), the Pt-PB(1:1)/C and Pt-PB(2:1)/C catalysts present massive agglomerated PB on carbon supports, which can hinder the electron transfer and thus the ORR kinetics because PB is a bad electron conductor. However, less PB ratio in the Pt-PB(8:1)/C catalyst may not form a uniform layer on carbon supports, which also leads to lower ORR kinetics by reducing its interaction with Pt NPs. The kinetic ORR current densities at various electrode potentials are calculated from the polarization curves in Figure 3b using the Koutecky−Levich equation, which is useful to obtaining the ORR kinetics, especially in the polarization region controlled by both kinetics and diffusion
Figure 2. (a) TEM image of Pt-PB(4:1)/C catalyst (inset is size distribution of Pt NPs). (b) HAADF-STEM image of Pt-PB(4:1)/C catalyst (inset is selected-area EDS mapping of Pt and Fe elements).
1 1 1 = + I Ik Id
(1)
where I, Ik, and Id are the experimental, kinetic and diffusionlimited current densities, respectively. The mass activities (mA mg−1Pt) and specific activities (mA cm−2Pt) for Pt-PB(4:1)/C, Pt/C-PDDA, and Pt/C catalysts are then reported with respect to the Pt loading and electrochemical activity surface area (ECSA), which is obtained based on the following equation
nm are uniformly distributed, while PB is not observed probably because of its membranous and amorphous nature. However, the XRD pattern of Pt-PB(4:1)/C catalyst shows the diffraction peaks of PB and Pt (Figure S6), illustrating the presence of PB in this composite catalyst. The composition and uniformity of the catalyst are further examined by high angle annular dark field scanning TEM (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS) (Figure 2b). The light spots in the HAADF-STEM image are in line with Pt element distribution in the EDS mapping, demonstrating the uniformity of Pt NPs distribution. Meanwhile, the Fe element is spread over the whole region, suggesting that the PB CNG-M-AM is uniformly dispersed or decorated on the surface of carbon support. Also, the scanning electron microscopy (SEM) image and EDS mapping results in a ∼70 × 70 μm region (Figure S7) confirm the dispersion uniformity of PB in the composite catalysts. Therefore, a large amount of boundaries of PB and Pt are formed, which are rather helpful to enhancing the catalytic properties of Pt NPs.
ECSA =
QH ϑC
(2)
where QH (V A) is the integrated area of the hydrogen desorption profiles in the CV curves in N2-saturated 0.1 M HClO4 solution (Figure S10), ϑ (V s−1) is the scanning rate, and C (210 μC cm−2) is the H desorption charge density on polycrystalline Pt. Figure 3c and Figure 3d show the mass and specific activities of the Pt-PB(4:1)/C, Pt/C-PDDA and Pt/C catalysts, respectively (detailed data are listed in Table S2). At 0.9 V, the mass activity of Pt-PB(4:1)/C catalyst (160.2 mA mg−1Pt) is 2.4 and 2 times that of Pt/C (66.5 mA mg−1Pt) and Pt/C-PDDA catalysts (80.5 mA mg−1Pt), whereas at 0.85 V, the 15252
DOI: 10.1021/acsami.6b02630 ACS Appl. Mater. Interfaces 2016, 8, 15250−15257
Research Article
ACS Applied Materials & Interfaces
Figure 3. (a) Ring current and (b) disk current density for Pt-PB/C, Pt/C, and Pt/C-PDDA catalysts in O2-saturated 0.1 M HClO4 solution (scanning rate: 5 mV s−1), and (c) kinetic mass activities and (d) specific activities of Pt/C, Pt/C-PDDA, and Pt-PB(4:1)/C catalysts at various potentials.
modified electronic structures (electronic effect).41 In our case, geometric effect is not the main possible reason for the activity enhancement, because the mechanical contact with PB is usually not sufficient to induce the rearrangement of surface Pt atoms. Moreover, the Pt-PB/C and Pt/C catalysts have similar Pt NPs before the electrochemical testing (Figure S12), which are confirmed by hydrogen absorption/desorption voltammograms (Figure S13), and after CV measurement (Figures S12 and S14). Meanwhile, the Pt 4f binding energies in the X-ray photoelectron spectra (XPS) of Pt-PB/C and Pt/C catalysts are almost the same (Figure S15), which rules out the possibility of electronic effect. Also, this activity enhancement cannot be due to the reduction of H2O2 production in the presence of PB, since all H2O2 yield is less than 0.5% (Figure 3a). Besides, the high activity of Pt-PB/C catalysts cannot be attributed to the intrinsic activity of PB CNG-M-AM, because PB alone can only catalyze the ORR at the potential of
