Immobilizing Pd nanoclusters into electronically conductive metal-organic frameworks as bi-functional electrocatalysts for hydrogen evolution and oxygen reduction reactions

Electrochimica Acta 306 (2019) 627e634

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Immobilizing Pd nanoclusters into electronically conductive metal-organic frameworks as bi-functional electrocatalysts for hydrogen evolution and oxygen reduction reactions Fuqin Zheng a, b, Chunmei Zhang a, b, Xiaohui Gao a, b, Cheng Du a, c, Zhihua Zhuang a, c, Wei Chen a, c, * a b c

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, China University of Chinese Academy of Sciences, Beijing, 100049, China University of Science and Technology of China, Hefei, Anhui, 230026, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 September 2018 Received in revised form 29 January 2019 Accepted 24 March 2019 Available online 26 March 2019

Although noble metals, especially Pt and Pt-based materials, are the most efficient catalysts for electrocatalysis, their high cost, low abundance and the strong tendency to aggregate of nanoparticles hinder their further commercial applications. In recent years, metal-organic frameworks (MOFs) have been found to be promising templates for immobilizing nanoclusters (NCs) as advanced catalysts. However, their poor electrical conductivities largely limit their applications in the field of electrochemistry and electrocatalysis. In this study, electrically conductive MOF-74 is chose as template to immobilize tiny Pd NCs in the pores (Pd@MOF-74). Unlike the usually reported MOF-based electrocatalysts, without the post calcination process, the Pd@MOF-74 exhibited excellent electrocatalytic performances for both hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). Moreover, due to the tiny size and the high dispersion of Pd clusters confined in the MOFs, the loading of noble Pd metal can be largely reduced for the catalysis. Meanwhile, the Pd clusters immobilized in the MOF-74 showed higher long-term durability in both acid and alkaline electrolytes than commercial Pt/C. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Metal-organic frameworks Electrocatalyst Hydrogen evolution reaction Nanocluster Oxygen reduction reaction

1. Introduction With the increasing global energy demand and diminishing source of fossil fuels, hydrogen molecule (H2), a renewable and clean fuel, is regarded as one of the most promising energy carrier for future energy infrastructure. Electrochemical hydrogen evolution reaction (HER) is a promising route for green water splitting, which gets rid of hydrocarbons as the source of hydrogen. On the other hand, the sluggish oxygen reduction reaction (ORR) on cathode is a critical issue to further improve the energy efficiency of fuel cells. Therefore, recognition of economical and superior HER and ORR electrocatalysts is of paramount importance to make fuel cell technology more viable alternative energy source [1,2]. Nevertheless, the low terrestrial abundance, prohibitive cost and

* Corresponding author. State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Changchun, Jilin, 130022, China. E-mail address: [email protected] (W. Chen). https://doi.org/10.1016/j.electacta.2019.03.175 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

strong tendency to aggregate of the most efficient Pt and Pt-based nanoparticle catalysts commonly used for such applications hinder their further applications. Therefore, exploring inexpensive alternatives to platinum electrocatalysts have attracted increasing attention and interest. Pd is regarded as a competitive alternative electrocatalyst because of its higher electrocatalytic activity and stability than those of 3d transition metals and their alloys [3e5]. At present, in order to improve the catalytic performance and lower the cost of Pd catalysts, an effective strategy is to design a support with unique structure for stabilizing Pd NPs and lowering the dosage of Pd, which can improve its atomic utilization. Metal-organic frameworks (MOFs) are highly porous crystalline materials assembled from metal ions or metal clusters coordinated to various organic linkers. MOFs often possess 3D extended microporous or mesoporous framework structures incorporating nano- and meso-sized pores, yielding large and accessible internal surface areas. In virtue of their extremely low density, large accessible pore volumes, tunable textures and well-defined pore size distributions, MOFs have attracted intensive interests in application in various research and industrial areas, such as gas

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storage and separation, catalysis, electrical devices, sensors, substrate recognition and drug delivery [6e10]. In addition, their tunable pore sizes and controllable pore environments based on the desired functionality and characteristics of the guest molecules make MOFs suitable candidates as templates for encapsulation of such as metal (M-), metal oxide (MO-) nanoparticles (NPs) or nanoclusters (NCs) with catalytic properties [11,12]. Furthermore, the heteroatom donors on the frameworks would effectively prevent the aggregation and migration of catalytically active NPs in solid state, which consequently make the NPs@metal-organic catalysts highly active and reusable. For example, Xu et al. introduced ultrafine Pt NPs into MOF nanopores (Pt NPs@MIL-101) without Pt NPs aggregation on the external surface of framework, which exhibited excellent catalytic performances for liquid-phase (ammonia borane hydrolysis), solid-phase (ammonia borane thermal dehydrogenation) and gas-phase (CO oxidation) reactions [13]. Jiang et al. synthesized mesoporous MOF-stabilized bimetallic Pd@Co coreeshell NPs with size smaller than the pores. The resultant Pd@Co NPs inside MOF pores showed synergistic and superior catalytic activity as well as excellent recyclability in hydrolytic dehydrogenation of NH3BH3 solution under mild conditions [14]. However, pristine MOFs usually exhibit poor electrical conductivity, which certainly limits their utility in a number of desirable electrochemical technologies, such as fuel cells, supercapacitors, thermoelectrics, especially electrocatalysis [8,15e17]. Furthermore, pristine MOFs suffer from instability in strong acidic or alkaline aqueous environments, which, however, are common in electrochemical devices. To realize the application of MOF-based materials in electrocatalysis, MOFs and their derivatives are usually calcinated at high temperatures under various atmospheres (N2, O2, N2/H2, etc.) to achieve updoped/doped carbonaceous nanostructures and metal/carbon (M/C) hybrids [18]. However, it has been found that dissolution and the collapse of channels could happen during the calcination process, which can destroy the ordered structure of MOFs. To the best of our knowledge, only few MOFs-based materials have been used as electrocatalysts without thermal treatment [19e23]. In recent years, there have been increasing interests in electrically conductive porous MOFs [24]. For example, Ni3(HITP)2 acts as a stand-alone ORR electrocatalyst with an onset potential of 0.82 V (with j ¼ 50 mA cm2) in 0.1 M KOH [23]. MOF-74 compounds are a series of materials with the formula of M2 (DOBDC) (M ¼ Mg, Mn, Fe, Co, Ni, Zn) and they have relatively robust framework with good thermal and chemical stability, making them good candidates for heterogeneous catalysts. The crystal structure of MOF-74-M is based on a honeycomb motif with pores of ~12 Å diameters, which are available to confine small metal nanoclusters. Furthermore, it has been reported that the conductivity of the guest-free MOF-74Fe frameworks is 4.8  108 S cm1, which is comparable to that of typical organic conductors [25]. In addition, pristine MOF-74-Co frameworks showed high electrocatalytic activity toward the electrochemical reduction of H2O2 [26]. In this work, by using the highly conductive M-MOF-74 (Co, Ni, Zn) as template, small Pd nanoclusters were encapsulated into the pores of the MOFs and their electrocatalytic activities for HER and ORR were investigated. As far as we know, only Chen et al. [3] synthesized Pd-doped metal-organic frameworks for HER. However, as usual the materials were used as electrocatalysts after annealed at 500  C for 4 h to obtain PdCo alloys wrapped in nitrogen-doped carbon (PdCo@CN). In this case, the MOFs structure may have been destroyed. In the present study, the initial Pd@MOF74 was directly used as electrocatalyst without post annealing treatment. This may be the first report of noble metal nanoclusters confined in electrically conductive MOF for HER and ORR applications.

2. Experimental 2.1. Synthesis of MOF-74-M (Co, Ni, Zn) MOF-74-M (Co, Ni, Zn) was prepared following the previously reported procedure with a slight modifications, as shown in detail in the supporting information [27e29]. 2.2. Synthesis of Pd@MOF-74-Co The Pd2þ@MOF-74-Co was prepared according to the previously reported double-solvent method (DSM) with a minor modification [30,31]. Typically, 53.2 mg of activated MOF-74-Co was suspended in 10 mL of anhydrous n-hexane, which acts as hydrophobic solvent. The obtained mixture was sonicated for about 1 h until it became homogeneous. After that, 15 ml of different concentrations of PdCl2 aqueous solution (0.5, 1.0 and 2.0 mol L1) was added dropwise under constant vigorous stirring. Subsequently, the resultant suspension was continuously stirred for another 2 h. The solid which settled down to the bottom of the sample vial was isolated from the supernatant by decanting and then dried under vacuum at 80  C overnight to obtain the Pd2þ@MOF-74-Co. The Pd2þ@MOF-74-Co composites were dispersed in 5 ml 0.01 M N2H4$H2O solution while vigorous stirring for 6 h. The obtained samples were washed with deionized water and collected by centrifugation, and dried at 80  C under vacuum. The synthesized samples are denoted as Pd@MOF-74-Co-1, Pd@MOF-74-Co-2 and Pd@MOF-74-Co-3, respectively. The Pd@MOF-74-M (Zn, Ni) hybrids were synthesized with the same procedure, just using MOF-74-Zn and MOF-74-Ni as templates, respectively. 2.3. Characterization High-resolution transmission electron microscopy (HRTEM) measurements were conducted on a JEM-2010 (HR) microscope operated at 200 kV. The studied sample was prepared by dropping ethanol dispersion of sample onto carbon-coated copper TEM grids using pipettes and dried under ambient condition. XRD patterns were collected by a D8 ADVANCE (BRUKER Germany) with Cu-Ka radiation (l ¼ 1.54 Å). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a VG Thermo ESCALAB 250 spectrometer (VG Scientific) operated at 120 W. The compositions of the products were obtained by using an inductively coupled plasma-atomic emission spectrometer (ICP-AES, X Series 2， Thermo Scientific USA). The Barrett-Emmett-Teller (BET) measurements were conducted on a Micromeritics ASAP 2020 System using automatic volumetric adsorption equipment. 2.4. Electrochemical measurements The whole electrochemical experiments were completed by using a CHI 750D electrochemical workstation with a standard three-electrode cell under the ambient atmosphere. The catalyst inks were prepared by mixing a composite catalyst (2 mg) and carbon black (CB, 4 mg) into the mixture of water, ethanol and isopropanol (v/v/v ¼ 3:1:1, 1 ml), which contains 5 ml of Nafion. The rotating disk electrode (RDE) or rotating ring-disk electrode (RRDE) coated with appropriate amount of catalyst inks was used as working electrode. A Pt coil and an Ag/AgCl with saturated KCl solution were used as counter electrode and reference electrode, respectively. Note that all the electrochemical data were given without any iR drop correction during the measurements. In this work, 0.5 M H2SO4 or 0.1 M KOH aqueous solution was used as electrolyte. The measured potentials (vs Ag/AgCl) in this work were

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converted to the RHE according to the Nernst equation:

ERHE ¼ EAg=AgC1 þ 0:197 þ 0:059pH

(1)

The Tafel slope for OER was calculated by the equation below:

E ¼ a þ b* log j

(2)

where E denotes the potential (vs RHE), b denotes the Tafel slope, j denotes the current density. The accelerated durability tests (ADTs) were performed by using cyclic voltammetry between 0 and 0.4 V (vs Ag/AgCl) in 0.5 M H2SO4 or 0.1 M KOH, aiming to evaluate the stability of the products. In addition, electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 10 Hz to 1 MHz in 0.1 M KOH, while the initial E and disturbance amplitude were set at 0 V and 5 mV, respectively. Nyquist plots (Z0 vs. Z'') were analyzed using Z-plot and Z-view software (Version 3.1, Scribnerassociates Inc., USA). The methanol tolerance of the samples was measured in 0.1 M KOH with the addition of 1.0 M methanol at t ¼ 400s. The kinetic current density (JK) can be calculated from RDE measurements by using the Koutecky-Levich equation:

1 1 1 ¼ þ J JK JL

(3)

where J is the experimentally obtained current density, JL refers to the measured diffusion-limited current density, and JK is the masstransport free kinetic current density. The JL term can be obtained from the Levich equation:

JL ¼ 0:201 nFCO DO n1=6 u1=2 2=3

(4)

where n is the number of electrons transferred; F is Faraday's constant (96,485 C mol1); CO is the concentration of molecular oxygen in 0.1 M KOH solution (1.2  103 mol L1); DO is the diffusion coefficient of O2 in 0.1 M KOH solution (1.9  105 cm2 s1); n is the kinematic viscosity of the electrolyte (0.01 cm2 s1) and u is the RDE rotation rate in rpm. The H2O2 yield (HO 2 %) and the electrons transfer numbers (n) per oxygen molecule are calculated from RRDE measurements using the following equations:

n¼

4ID ID þ IR =N

HO 2% ¼

200IR =N ID þ IR =N

(5)

(6)

where IR and ID are ring and disk currents, and N is collection efficiency (0.37).

3. Results and discussion 3.1. Materials preparation and characterization MOF-74-Co was prepared following the procedure reported previously with a small modification, which is described in detail in the Experimental Section (Supporting Information). Powder XRD patterns indicated that the synthesized MOF-74-Co has a good crystal structure identical to the reference (Fig. S1a). In this work, we aim to synthesize electrically conductive MOF as template to confine metal nanoclusters for electrocatalysis with no need of the post-treatment of high-temperature calcination. Here,

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electrochemical impedance spectroscopy (EIS) measurements were first performed and the results showed that the prepared MOF-74 templates have excellent electrical conductivities with Rct lower than 50 U, as shown in Fig. S1b. Such electronic conductivities are higher than those of the recently reported conductive MOFs [24,32]. To further demonstrate the electrical conductivity of the materials, the EIS of MOF-74 films were also measured at open circuit potential in 0.1 M KCl with the presence of 5 mM K3Fe(CN)6, as shown in Fig. S1c. The corresponding cyclic voltammograms (CVs) are shown in Fig. S1d. The EIS spectra indicate that these MOFs have intrinsic high electrical conductivity with Rct about 50 U. Meanwhile, [Fe(CN)6]3-/[Fe(CN)6]4- shows a quasi-reversible redox reaction on the MOF-74 with comparable peak potential difference of 120 mV and higher peak current compared to bare GCE, further confirming the high electron transfer and conductivity of the prepared MOFs. The pore structure of the MOF-74 was examined by the nitrogen adsorption/desorption analysis. Based on the isothermal curve shown in Fig. S2, the MOF-74-Co shows a very large BET surface area (801.18 m2 g1) and pore volume (0.354 cm2 g1) with a mean pore size of 1.77 nm. The high conductivity and abundant pore structure render the present MOF-74Co a good template for electrochemical application. The preparation of Pd nanoclusters (Pd NCs) encapsulated in MOF-74-Co (denoted as Pd@MOF-74-Co) is depicted in Scheme 1. Pd@MOF74-Co was prepared with a double-solvent impregnation method (DSM) followed by a reducing process [13,14,30,31]. For the impregnation process, activated MOF-74-Co was suspended in a large amount of anhydrous n-hexane, followed by the dropwise addition of PdCl2 aqueous solution under vigorous stirring. It was found that excessively used hydrophobic hexane can overspread the external surface of the MOF, which ensures the diffusion of hydrophilic PdCl2 into the pores of MOF-74-Co via capillary force. The subsequent hydrazine hydrate reduction process resulted in the formation of Pd nanoclusters encapsulated in the cavity of MOF-74-Co (Pd@MOF-74-Co). The similar Pd@MOF-74-M (Zn, Ni) hybrids were also synthesized in the same way. As shown in Fig. 1, the XRD patterns of the as-prepared Pd@MOF-74-Co samples are similar to that of the parent MOF-74Co, suggesting that the crystalline structure of the MOF-74-Co template is still well retained after the encapsulation of Pd NCs. However, due to the small size of the Pd NCs, only very weak diffraction peak from Pd (111) can be observed in the XRD patterns of Pd@MOF-74-Co. A little clearer Pd (111) peaks can be seen in the XRD patterns of Pd@MOF-74-M (Zn, Ni) (Figs. S3 and S4). The morphologies of the MOF-74-Co and Pd@MOF-74-Co were characterized by SEM. As shown in Fig. 2a and b, before and after the confinement of Pd NCs, both samples exhibit rod-like shape with a diameter of ~5 mm and length in tens of micrometers. The TEM image shown in Fig. 2c reveals that highly dispersed Pd NCs with an average diameter of 3.0 nm have been confined in the cavities of the MOF-74-Co. The size of the Pt NCs shows a little larger than that of the pore size of MOF-74 (1.77 nm), which has been commonly observed for metal@MOFs [33,34]. Further HRTEM measurement (Fig. 2d) indicates that the formed Pd NCs show two types of exposed crystal facets with the inter-planar distances of 0.223 and 0.194 nm, which are ascribed to the Pd (111) and Pd (100) facets. The amount of encapsulated Pd NCs in Pd@ MOF-74-Co was determined by inductively coupled plasma optical emission spectrometer (ICP-OES) (Table S1). In addition, XPS was performed to get the surface chemical information of the Pt@MOF-74-Co. The XPS survey spectrum of Pd@MOF-74-Co shows the presence of Co, Pd elements (Fig. S5). The deconvoluted XPS of Pd 3d displays two sets of peaks (Fig. 3a), corresponding to the Pd0 and Pd2þ, respectively, further demonstrating the successful encapsulation of Pd NCs in the MOF-74-Co cages. Meanwhile, Co2þ is still kept in the

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Scheme 1. Synthetic process and application of Pd@MOF-74.

Fig. 3. XPS spectra of the obtained Pd@-MOF-74-Co sample: a) Pd3d high-resolution spectrum, and b) Co 2p high-resolution spectrum.

Fig. 1. XRD patterns of the as-prepared MOF-74-Co and Pd@ MOF-74-Co samples.

framework of MOF-74 (Fig. 3b). For comparison, MOF-74-Ni and MOF-74-Zn were also used as templates to synthesize Pd NCs. As shown in Figs. S3eS8, similar Pd

Fig. 2. SEM images of a) MOF-74-Co and b) Pd@MOF-74-Co; c, d) TEM images of Pd@MOF-74-Co.

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NCs can be formed in the MOF-74-M (Ni, Zn) frameworks. From the HRTEM images, the crystal facets of Pd (111) and Pd (100) can be observed and two valence states of Pd0 and Pd2þ are present as shown in the XPS measurements. The average diameter of the Pd nanoclusters is also 2e3 nm for both samples (Fig. S6). 3.2. Electrocatalytic performance of Pd@MOF-74-Co towards the HER To evaluate the electrocatalytic activity of the Pd NCs for HER, the synthesized catalysts were deposited on a glassy carbon rotating disk electrode (RDE) with a fixed mass loading of 0.16 mg cm2. The HER activities of the as-prepared electrocatalysts were measured in 0.5 M H2SO4 electrolyte with linear sweep voltammetry (LSV) at 1600 rpm and a scan rate of 10 mV s1. Fig. 4a compares the LSV curves of HER on the Pd@MOF-74-Co samples with different Pd loadings. It can be seen clearly that all the Pd@MOF-74-Co samples display much higher HER catalytic activities than the template of MOF-74-Co, indicating that the electrocatalytic active sites are mainly from the confined Pd clusters. Meanwhile, as shown in Fig. 4a and Table S1, the HER activity of Pd@MOF-74-Co increases with increasing the Pd content and the Pd@MOF-74-Co-3 shows the highest HER activity with onset potential positively shifted to 40 mV. Fig. 4b compares the Tafel plots of the three Pd@MOF-74-Co samples and the Pd@MOF-74-Co-3 has the smallest Tafel slope of 57 mV$dec1, suggesting the fast hydrogen evolution reaction on the Pd@MOF-74-Co-3. The HER catalytic activity of Pd@MOF-74-Co-3 was then compared with the commercial Pt/C catalyst (20 wt%). As shown in Fig. 4c, the Pd@MOF-74-Co-3 displays a comparable onset potential (40 mV) to the commercial 20% Pt/C (32 mV) catalyst. Moreover, the current densities of HER on Pt@MOF-74-Co-3 are obviously larger than those obtained from the commercial Pt/C. For example, at the potentials of 0.076, 0.106, 0.145 and 0.197 V (vs RHE), the current densities obtained from the Pd@MOF-74-Co-3 are 5, 10, 20, 40 mA cm2 mg1 respectively, which are around 1.88, 1.7, 1.93 and 2.45 times higher than those from the commercial Pt/C (Fig. 4d). These results indicate that the Pd@MOF-74-Co hybrid has improved HER catalytic performance and meanwhile, the usage of

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noble metals can be largely reduced. The electrochemical stability of the Pd@MOF-74-Co-3 was also evaluated by accelerated durability tests (ADTs). As shown in Fig. 4f, after 2500 potential cycles test, the polarization curve of HER from the Pd@MOF-74-Co-3 is still well retained, while the HER on commercial Pt/C shows much decreased current densities and increased overpotential. For example, after 2500 cycles, to obtain a current density of 10 mA cm2 mg1 on commercial Pt/C, the corresponding potential shifts negatively from 0.142 to 0.228 V. These stability tests clearly indicate that the prepared Pd@MOF-74-Co-3 has high durability and strong corrosion-resistance. In addition, the HER catalytic activities of Pd NCs confined in MOF-74-Zn and MOF-74-Ni were also investigated. The electrochemical measurements shown in Figs. S9a and S11a indicate that all the Pd@MOF-74-M (Zn, Ni) electrocatalysts have higher electrocatalytic activities than the support templates for HER. The optimized samples of Pd@MOF-74-Zn-2 and Pd@MOF-74-Ni-3 exhibit the onset potentials of 35 and 41 mV for HER, which are much close to that of commercial Pt/C. It should be noted that similar to the Pd@MOF-74-Co-3, these two Pd@MOF-74-Zn-2 and Pd@MOF-74-Ni-3 samples also exhibited much larger HER current densities than commercial Pt/C, as shown in Figs. S10 and S12. ADTs indicate that these two samples also have higher durability and stronger corrosion-resistance than commercial Pt/C (Figs. S10d and S12d). However, from Fig. S13, one can see that the Pd@MOF-74-Co3 has the best HER activity among the three Co-, Zn- and Ni-based MOF derivatives. This may be resulted from the combined effects of metal species in MOFs, content of Pd NCs, the pore volume etc., which needs to be further investigated. To further evaluate the catalytic activities of the prepared catalysts for HER, the Tafel plots on the different samples were measured and compared (Fig. 4e, S10c and S12c). Normally, smaller Tafel slope implies the faster HER rate [35]. The Tafel slope of 57 mV$dec1 obtained from Pd@MOF-74-Co-3 (Fig. 4e) suggests that the electrochemical desorption process, i.e. the Heyrovsky reaction is the rate determining step for HER. By contrast, the Tafel slope of 34 mV$dec1 from commercial Pt/C reveals that the Tafel recombination step is the dominated process [36]. These results show that two different mechanisms are involved for HER on these

Fig. 4. (a) Polarization curves of the Pd@MOF-74-Co and MOF-74-Co for HER in 0.5 M H2SO4, with a potential scan rate of 10 mV s1. (b) Tafel plots from Pd@MOF-74-Co. (c) Polarization curves of the Pd@MOF-74-Co-3 and Pt/C for HER in 0.5 M H2SO4, with a potential scan rate of 10 mV s1. (d) Comparison of current densities from Pd@MOF-74-Co-3 and Pt/C at different potentials. (e) Tafel plots from Pd@MOF-74-Co-3 and Pt/C. (f) Polarization curves of the Pd@MOF-74-Co-3 and commercial Pt/C after 2500 cycles test in 0.5 M H2SO4.

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two types of catalysts. Meanwhile, Tafel slopes of 63 and 80 mV$dec1 were obtained from the optimized Pd@MOF-74-M (Ni, Zn) samples (Figs. S9 and S11), respectively, which indicate that HER on the Pd@MOF-74-M (Co, Ni, Zn) has a similar reaction mechanism.

3.3. Electrocatalytic performances toward the ORR The catalytic activities of the different MOF-defined Pd NCs for the ORR were evaluated by depositing the samples onto a rotating GC electrode with a catalyst loading of 0.16 mg cm2 in 0.1 M KOH aqueous electrolyte. Fig. 5a and Fig. S14 show the CV curves of the studied samples and Pt/C in N2- and O2-saturated 0.1 M KOH. It can be seen that the three MOF-based composites exhibited featureless CV curves in N2-saturated electrolyte, whereas prominent cathodic oxygen reduction peaks are observed when the electrolyte solution was saturated with O2, suggesting their effective catalytic activities toward ORR. Fig. 5b compares the linear sweep voltammetry (LSV) curves of Pd@MOF-74-Co with different Pd loading and the commercial Pt/C catalyst (20 wt %) at a rotation speed of 1600 rpm. As anticipated, compared with the initial MOF-74-Co, the formation of Pd nanoclusters in the template (Pd@MOF-74-Co) can improve significantly the ORR activity. Meanwhile, the ORR catalytic activity of the Pd@MOF-74-Co is dependent on the mass loading of Pd and

the optimized sample of Pd@MOF-74-Co-3 shows the highest ORR activity with an onset potential of 0.938 V (vs. the RHE) and a halfwave potential of 0.798, which are almost same to those of Pt/C (0.938 and 0.800 V, respectively) (Fig. 5b). Similar to Pt/C, the Pd@MOF-74-Co-3 exhibited a well-defined plateau of limited current density of about 5.3 mA cm2 in O2-saturated 0.1 M KOH solution starting from 0.6 to 0.2 V. According to the Koutecky-Levich (K-L) plots derived from the RDE measurements at different potentials (Fig. 6 and Fig. S15), the electron transfer number (n) of ORR on Pd@MOF-74-Co-3 is 4.0, indicating an efficient 4e-dominated ORR pathway. In the rotating ring-disk electrode (RRDE) measurements (Fig. 6c and S16), the onset potential of the ORR on Pd@MOF-74-Co-3 is comparable to that on Pt/C. The negligible HO 2 yield (less then 16%) clearly confirms the superior ORR catalytic efficiency of Pd@MOF-74-Co-3 and the calculated n numbers of 3.6e3.9 for Pd@MOF-74-Co-3 at different potentials further suggest the efficient four-electron ORR process (Fig. 6d). Fig. 5c shows the Tafel plots of ORR on different electrocatalysts. The Tafel slopes of Pd@MOF-74-Co-1, Pd@MOF-74-Co-2 and Pd@MOF-74-Co-3 were calculated to be 67, 59 and 66 mV dec1, respectively, which are smaller than that of Pt/C (72 mV dec1). The results reveal that all the three Pd@MOF-74-Co hybrid products show very good ORR kinetic processes with faster oxygen reduction rate than commercial Pt/C catalyst [37,38].

Fig. 5. (a) CV curves of Pd@MOF-74-Co-3 and 20% Pt/C in nitrogen- and oxygen-saturated 0.1 M KOH solution with a scan rate of 100 mV s1. (b) Positive-going ORR polarization curves recorded on different samples in an O2-saturated 0.1 M KOH solution with a sweep rate of 10 mV s1 and a rotation rate of 1600 rpm. (c) The corresponding Tafel plots. (d) Noble metal mass activities of Pt/C and Pd@MOF-74-Co-3 at 0.8 and 0.85 V before and after 2500 potential cycles test. (e) Comparison of positive-going ORR polarization curves obtained from Pd@MOF-74-Co-3 and commercial Pt/C before and after durability tests. (f) Comparison of the chronoamperometric response over 1000 s in O2-saturated solution at 0.7 V for Pd@MOF-74-Co-3 and Pt/C catalysts in 0.1 M KOH (the arrow indicates the addition of methanol).

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Fig. 6. (a) RDE polarization curves of ORR on Pd@MOF-74-Co-3, with a potential scan rate of 10 mV s1. (b) The corresponding K-L plots of ORR on Pd@MOF-74-Co-3 at different potentials. (c) RRDE measurements of oxygen reduction on Pd@MOF-74-Co-3 and commercial Pt/C in O2-saturated 0.1 M KOH with a scan rate of 10 mV s1 and rotation speed of 1600 rpm. (d) The corresponding H2O2 yield (HO 2 %) and electron transfer number (n) on Pd@MOF-74-Co-3 and commercial Pt/C.

The noble metal (Pt or Pd) mass activity of the Pd@MOF-74-Co-3 was then compared with the state-of-the-art commercial Pt/C, as shown in Fig. 5d. At 0.8 V, the noble metal mass activities of Pt/C and Pd@MOF-74-Co-3 are 0.091 and 0.721 mA mg1, respectively. The mass activity of the Pd@MOF-74-Co-3 is 8-fold higher than that of Pt/C. A similar enhancement (9.4-fold) was also observed at 0.85 V. These results indicate that the Pd@MOF-74-Co hybrid has much higher ORR mass activity than commercial Pt/C. Another important factor for the practical application of a catalyst is its longterm durability. Thus, the durability of Pd@MOF-74-Co-3 was evaluated by an accelerated durability test (ADT) at room temperature. As shown in Fig. 5e, negligible shift of the ORR polarization curves can be observed for the Pd@MOF-74-Co-3 catalyst before and after 2500 potential cycles in O2-saturated electrolyte. However, the polarization curve of Pt/C catalysts shows an obvious negative shift after the 2500-cycle ATD. After the ADT, the mass activity of Pd@MOF-74-Co-3 at 0.8 V still maintains 0.720 mA mg1 (Fig. 5d). By contrast, the commercial Pt/C catalyst lost 25.3% of its initial mass activity (decreased from 0.091 to 0.068 mA mg1). These results demonstrate that the Pd@MOF-74-Co-3 has excellent longterm durability. The enhanced stability can be ascribed to the confinement of Pd clusters in the porous MOF framework although there is no any protecting ligand capped on the cluster surface. Furthermore, the methanol tolerance of Pd@MOF-74-Co-3 was also evaluated. As shown in Fig. 5f, both Pd@MOF-74-Co-3 and Pt/C catalysts show excellent ORR activity. However, after the injection of 1.0 M method into the electrolyte (t ¼ 400s), an obvious methanol oxidation peak can be observed from the commercial Pt/C catalyst, indicating the poor tolerance of Pt/C to methanol. In contrast, there is no obvious ORR performance decay on the Pd@MOF-74-Co-3 upon the addition of 1.0 M methanol, implying the excellent tolerance of Pd@MOF-74-Co-3 to methanol crossover. Meanwhile, the ORR catalytic properties of Pd@MOF-74-M (Zn, Ni) were also studied, as showed in Figs. S17eS24 and Table S2. We can find that similar results can be obtained from these two types of MOF-Pd cluster composites; that is, the confined Pd NCs in the

MOF-74 can largely enhance the ORR activity, and the ORR performance of Pd@MOF-74-M (Zn, Ni) increases with increasing the mass loading of Pd. In addition, as shown in Figs. S17c and S20c, the ORR mass activities of Pd@MOF-74-Zn-3 and Pd@MOF-74-Ni-3 showed 4.8 and 5.2-fold higher than that of Pt/C at 0.8 V, respectively (after 2500 CV cycles, the activities increase to 7.1 and 6.7fold, respectively). Besides, the optimized samples of Pd@MOF74-Zn-3 and Pd@MOF-74-Ni-3 possess high durability after 2500 potential cycles without obvious change of onset potential and halfwave potential, which is much better than the Pt/C. Moreover, both Pd@MOF-74-Zn-3 and Pd@MOF-74-Ni-3 exhibit excellent tolerance to methanol crossover, as shown in Fig. S23. These electrochemical results indicate that the optimized Pd@MOF-74-M (Co, Zn, Ni) composites have excellent ORR performances and meanwhile the usage of precious metals is largely reduced. 4. Conclusion In summary, different from the previous related studies, in this work, highly conductive MOFs were used as effective templates for confinement of precious metal clusters and the prepared composites can be used directly as electrocatalysts with no need of posttreatments of pyrolysis and carbonization due to the good conductivity. Because of the tiny size, clean surface, high dispersion of the formed Pd nanoclusters, and the effective protection of MOF framework, as a type of bifunctional electrocatalysts, the synthesized Pd@MOF-74-M (Co, Zn, Ni) hybrids exhibited excellent catalytic activities and stabilities for both HER and ORR. Meanwhile, by dispersing Pd nanoclusters in the cavities of MOFs, the usage of noble metals can be largely reduced. The Pd@MOF-74-Co-3 demonstrated 8-fold (at 0.8 V) and 9.4-fold (at 0.85 V) higher mass activities than Pt/C for ORR. The present study not only provides a strategy for synthesizing confined metal nanocrystals with promising application in electrocatalysis but also demonstrates the promising application of conductive MOFs in electrocatalysis. Meanwhile, due to the high catalytic activity, durability and high

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