Fabrication of Pd [email protected] catalysts for highly efficient electrocatalytic sensing of H2O2 and high-performance supercapacitor

Materials and Design 186 (2020) 108267

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Fabrication of Pd Nanocubes@CdIF-8 catalysts for highly efficient electrocatalytic sensing of H2O2 and high-performance supercapacitor Yue Li a, Lanshu Xu a, MengYing Jia a, Linlin Cui a, Xinyan Liu b, ∗∗, XiaoJuan Jin a, ∗ a b

MOE Engineering Research Center of Forestry Biomass Materials and Bioenergy, Beijing Forestry University, 35 Qinghua East Road, Haidian, 100083, Beijing, China Institute of Vegetables and Flowers, Chinese Academy of Agricultural Sciences, Beijing, 100081, PR China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• The Pd@Cd(2-methylimidazole)2 was synthesized by two facile methods with original cubic Pd nanoparticles anchored on the CdIF-8 which supported on the acidified activated carbon. • The A-Pd0.2Cd1.5IF-8@AAC and BPd0.2Cd1.5IF-8@AAC sensors demonstrated a linear response to H2O2 in the range of from 10 μM to 10.2 mM and 2 μM to 17.5 mM. • The asymmetric supercapacitor has been also fabricated which exhibites a definite improvement on electrochemical properties with high specific capacitance, excellent stability and prominent energy density.

a r t i c l e

i n f o

Article history: Received 5 June 2019 Received in revised form 8 October 2019 Accepted 9 October 2019 Available online 24 October 2019 Keywords: Acidified activated carbon A(B)-PdxCdyIF-8@AAC Electrochemical sensor Asymmetric supercapacitor

a b s t r a c t In this work, the Pd@ Cd(2-methylimidazole)2 (Pd@CdIF-8) was synthesized by two facile methods, in which the original cubic Pd nanoparticles anchored on the surface/interface of CdIF-8 and supported on the acidified activated carbon (AAC). Interestingly, CdIF-8 exhibited sphericity under hydrothermal condition and presented hexahedron under mild liquid. A(B)-PdxCdyIF-8@AAC hybrids are researched by FESEM, TEM, contact angle measurement, XPS, XRD and BET analytical techniques. A(B)-PdxCdyIF-8@AAC hybrids are employed as nonenzymatic sensors for electrochemical detection of H2O2, and both exhibited excellent electrocatalytic activity, especially for A(B)–Pd0.2Cd1.5IF-8@AAC/GCE sensors. A(B)–Pd0.2Cd1.5IF-8@AAC/GCE sensors demonstrated a linear response to H2O2 in the range of from 10 μM to 10.2 mM and 2 μM to 17.5 mM, the sensitivity of 0.195 μAμM−1 and 0.266 μAμM−1, respectively. In addition, asymmetric supercapacitors have been fabricated using A(B)PdxCdyIF-8@AAC hybrids as positive electrode and exhibited exceptional electrochemical properties with high specific capacitance (452 F g−1 at 0.05 A g−1 and 372 F g−1 at 1 A g−1 of A-Pd0.2Cd1.5IF-8@AAC, and 502 F g−1 at 0.05 A g−1 and 395 F g−1 at 1 A g−1 of B–Pd0.2Cd1.5IF-8@AAC), excellent stability (83.579% maintenance of specific capacitance at 5 A g−1 of A-Pd0.2Cd1.5IF-8@AAC and 78.313% maintenance of B–Pd0.2Cd1.5IF-8@AAC), respectively. © 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

∗ Corresponding author. ∗∗ Corresponding author. E-mail addresses: [email protected] (X. Liu), [email protected] (X. Jin).

https://doi.org/10.1016/j.matdes.2019.108267 0264-1275/© 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Y. Li et al. / Materials and Design 186 (2020) 108267

1. Introduction Hydrogen peroxide (H2O2) is a strong and environmentally safe oxidant that has been widely used in many practical applications, such as asbiology, food industry, pharmaceutical and environmental protection areas[1]. Furthermore, H2O2 as a endogenously enzymatic byproduct of oxidases, can regulate biological signaling transduction processes and can function as an indicator to trace many biochemical reactions[2,3]. An appropriate level of H2O2 is beneficial for normal body function. However, an excessive accumulation of H2O2 in the human body could cause many diseases such as cell apoptosis, protein synthesis and DNA damage, ect. Therefore, quantitative and selective detection of the level of H2O2 is of great value in biological system[4–6]. Up to date, spectrophotometry, colorimatry, titrimetry, fluorescence, chemiluminescence and electrochemical methods have been investigated to determine the H2O2. Among them, electrochemical method, as an affordable and effective way, has sparked great research interests for the detection of H2O2 in vivo and vitro due to its excellent selectivity, intrinsic simplicity and high sensitivity. Conventional electrochemical H2O2 sensors are based on enzymatic and non-enzymatic electrochemical electrodes. Enzyme-based electrochemical sensors are more selective than non-enzymatic electrodes due to their specific for the substrates. Nevertheless, these sensors have seriously limited their applications due to insufficient stability, complicated immobilization procedure and high costs. Compared to enzymatic sensors, the non-enzymatic H2O2 sensors based on noble metals (Pt, Ag, Pd ect.) [7,8] and exceptional construction (ZIF-8, ZIF-67, MOF-5 ect.) [9–11] and their substrate (carbon-based materials) could boost the electrocatalytic activity toward H2O2 redox reaction and exhibit evident superiority. As a promising class of porous materials, Metal organic frameworks (MOFs), an emerging kind of microporous materials consisted of metal ions/clusters and rigid organic linkers, have generated intense interest over the past two decades due to their tailorable structures, crystalline nature and multifunctional applications in catalysis, adsorption, sensing and gas storage[12–18]. For example, Muxin Lu et al. have designed a graphene aerogel (GA) and metal-organic framework (MOF) hybrids for the determination of multiple heavy metal ions in aqueous solutions[19]. Juan Dai et al. have fabricated a novel microporous nanocomposites of ZIF-8 on multiwalled carbon nanotubes for adsorptive removing benzoic acid from water[20]. Thus, MOFs are demonstrated as homogeneous and heterogeneous catalysts. The high porosity and large surface area of MOFs contribute to host and support catalytically active metal nanoparticles. Recently, intense attention has generated on metal nanoparticles@MOF due to the great enhancement of catalytic performance through confining and stablizing the metal nanoparticles inserted into the interface or anchored on the surface of MOF. Shuaishuai Ding et al. have fabricated Pd@ZIF-8 catalysts with different Pd spatial distributions and their catalytic properties[21]. Hong Jiang et al. have designed Pd@ZIF-8 with different molar ratios of Pd/Zn2+ via an assembly method to enhance the catalytic performance of Pd@ ZIF-8[22]. Researches on the consequences for catalytic properties and functionalities of the active metal sites are unsatisfactory by reasons of their low electronic conductivity and insability in water medium. In addition, previous researches have reported that MOF-carbons hybrids could exhibit tremendous potential to enhance the physicochemical performances and circumvent drawbacks of MOFs, such as Hongying Zhao et al. have fabricated a novel bifunctional MOF (2Fe/Co)/carbon aerogel to enhance H2O2 and ·OH generation in solar photo-electroFenton process [6]. Electrochemical sensors based on activited carbon are outstanding hybrids due to its high surface area, unique electronic conductivity, excellent mechanical and thermal stability[23,24]. Espeially, the acidified activated carbon (AAC) with carboxyl, hydroxyl and epoxy is hydrophilic and can be easily functionalized through covalent and noncovalent interactions. Herein, our groups have rationally designed a representative A(B)-Pd x CdyIF-8@AAC

with Pd nanoparticles inserted into the interface and anchored on the surface of CdIF-8 and the Pd x Cd y IF-8 supported on the AAC. Pd nanoparticles exhibited high activity during the oxidationreduction reactions. Nonetheless, Pd nanoparticles exhibited poor stability and easily aggregated. The addition of CdIF-8 could provide abundant surface area and electrochemical active sites for the redox reaction, promote the stability of Pd and increase the yield of the H 2 O 2 , thus enhancing electrocatalytic abilities of PdCdIF-8 systems. However, the hydrophilicity and aggregation of PdCdIF-8 systems in water made them impossible to fully contact the detection object, and thus leading to limited electrocatalytic abilities [25–28]. The AAC as substrate in Pd x Cd y IF-8@AAC with high specific surface area showed excellent electrocatalytic abilities and remarkable physical properties. The Pd xCd y IF-8 successfully anchored on AAC (Pd x Cdy IF-8@AAC) via stable linkages. The AAC was chosen to form coordination interactions between hydroxyl and carboxyl groups of AAC and the charged of metal ions in CdIF-8. The significant host-guest interactions include the hydrogen interaction, the hydrophobic interactions and the acid-based interactions. Furthermore, the PdxCdyIF-8 anchored on the surface of AAC (which attributes to π-π packing and hydrogen bonding) to fabricate a hierarchical structure. The Pdx Cd y IF-8 could offer biomimetic catalytic and adsorption affinity for H2 O2 due to its Lewis basic sites and open metal sites. Therefore, it is significant for MOF to combine with other nanomaterials and broaden its applications in many fields, especially in the electrochemistry. To obtain the stable incorporation of Pd x CdyIF-8@AAC, our groups have investigated the chemical environment by using hydrothermal and liquidus approaches and controled the structure with different mass ratio of Pd and Cd 2+ to enhance catalytic selectivity and activity to detect H2O2. Interestingly, cubic Pd nanoparticles inserted into the interface and anchored on the surface of CdIF8 and CdIF-8 exhibited spherical under hydrothermal condition and presented hexahedron shape in gentle liquid phase with supported on the AAC modified glassy carbon electrode (GCE). The as-obtained A(B)-PdxCdyIF-8@AAC hybrids showed excellent electrocatalytic activity toward the determination of H 2 O2 , especially the A-Pd 0.2 Cd 1.5 IF-8@AAC and B–Pd 0.2 Cd 1.5 IF-8@AAC sensors. Under optimum conditions, the constructed A-Pd 0.2 Cd 1.5 IF-8@ AAC and B–Pd 0.2 Cd 1.5 IF-8@AAC sensors demonstrated a linear response to H 2 O 2 in the range of from 10 μM to 10.2 mM and 2 μM to 17.5 mM, the sensitivity of 0.195 μAμM−1 and 0.266 μAμM−1, respectively. The obtained results showed that the novel A(B)– Pd0.2Cd1.5IF-8@AAC sensors could be applied for the determination of H 2 O 2 in real samples. Interestingly, the as-prepared nanocomposites are also adopted in the asymmetric supercapacitors. The asymmetric supercapacitors were also fabricated with A(B)PdxCdyIF-8@AAC hybrids as the positive electrode and porous activated carbon as negative electrode. The electrochemical measurements showed that asymmetric supercapacitors also exhibited remarkable electrochemical performances.

Table 1 Compositions of A(B)-PdxCdyIF-8@AAC nanocrystals. Sample

Activited carbon content (g)

Pd/Cd2+ molar ratio

2-methylimidazole content (mmol)

Methanol content (mL)

A-CdIF-8@AAC A-Pd0.1Cd1.5IF-8@AAC A-Pd0.2Cd1.5IF-8@AAC A-Pd0.3Cd1.5IF-8@AAC B–CdIF-8@AAC B–Pd0.1Cd1.5IF-8@AAC B–Pd0.2Cd1.5IF-8@AAC B–Pd0.3Cd1.5IF-8@AAC

0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

0 1/15 2/15 1/5 0 1/15 2/15 1/5

4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5

90 90 90 90 90 90 90 90

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Fig. 1. FESEM and TEM images of A(B)-PdxCdyIF-8@AAC nanocomposites: (a-c): FESEM images of A-CdIF-8@AAC nanocomposites, (d-i): FESEM images of A-Pd0.2Cd1.5IF-8@AAC,(j-o): FESEM images of B–Pd0.2Cd1.5IF-8@AAC (p-r): TEM images of B–Pd0.2Cd1.5IF-8@AAC, (s-u): TEM images of A-Pd0.2Cd1.5IF-8@AAC.

2. Experimental section

grade and used directly without further purification. Double distilled water was used for the preparation of all the required solutions.

2.1. Material preparation 2.2. Characterization Cadmium nitrate, PdCl2, 2-methylimidazole and methanol, potassium dihydrogen phosphate (KH2PO4) and disodium hydrogen phosphate (Na2HPO4) were all purchased from Beijing Lanyi Chemical reagent. The precursor of activated carbon was kindly provided by Beijing Jiahekailai Furniture and Design Company, which was obtained during the furniture manufacturing process containing 12% of ureaformaldehyde resin adhesive. All chemicals were of analytical

The surface microstructure and morphologies of the hybrids were investigated using the field emission scanning electron microscope (FESEM, Japan, JSM-7001F) and the transmission electron microscopy (TEM, Netherlands, TecnaiTF20). The Brunauer-Emmett-Teller (BET) was used to evaluate the specific surface area. X-ray diffraction (XRD) spectra of hybrids were obtained by using the D/max-2550

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The electrochemical system including a glass carbon electrode (GCE) as the working electrode, an Ag/AgCl reference electrode and a platinum wire counter electrode, was employed for the electrochemical measurement with a CHI660D electrochemical workstation (Shanghai Chenhua). The phosphate buffer solution (PBS, pH = 9) used for electrochemical experiment was treated with nitrogen for 20 min. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were conducted to characterize the different A(B)-PdxCdyIF-8@AAC/GCE electrodes. The potential of CV and DPV were carried out according to the different electrodes in phosphate buffered solution (PBS 0.1 mol/L pH = 9). Hydrogen peroxide (H2O2) was diluted to a working concentration range for electrochemical detection. All electrochemical experiments were conducted at room temperature. The electrochemical capacitive performances of asymmetric supercapacitor were investigated using the CHI 660D electrochemical workstation and BT2000 battery testing system in 7 M KOH aqueous electrolyte in a two electrode cell at room temperature. 2.3. Preparation of the AAC The waste fiberboard, which was obtained in the furniture manufacturing process containing 10% ureaformaldehyde resin adhesive of the mass, was carbonized in a high-purity N2 at the temperature increase rate of 10 °C/min to the final temperature of 500 °C and maintained for 2 h. Then the obtained products were activated and mixed with KOH at the mass ratio of 3:1 under the temperature of 750 °C for 60 min in oven. After that, the prepared activated carbon was oxidized using the mixture of sulfuric and nitric acid as oxidization agent. Briefly, 3 g of activated carbon powder was mixed with 100 mL sulfuric and nitric acid solutions in round-bottom flask, and the solution was back flowed for 8 h under the temperature of 140 °C in the oil bath. After natural cooling to room temperature, the solution was centrifuged at 8000 rpm for 30 min to remove the large amount of agglomerated particles. During the period, the distilled water was changed for several times to neutral. 2.4. Synthesis of A(B)-PdxCdyIF-8@AAC Two different methods described below were carried out separately to fabricate different A(B)-PdxCdyIF-8@AAC materials to investigate the feasibility of the controlled synthesis for A(B)-PdxCdyIF-8@AAC hybrids.

Fig. 2. (a): Pore size distribution for all samples (b): Nitrogen adsorption isotherms (c): TGA images of A(B)-PdxCdyIF-8@AAC hybrids (d): XRD spectra of A(B)-PdxCdyIF-8@AAC hybrids (e): Cd 3d of A(B)-PdxCdyIF-8@AAC hybrids (f): Pd 3d of A(B)-PdxCdyIF-8@AAC hybrids.(g): contact angle of A-Pd0.2Cd1.5IF-8 catalyst (h): contact angle of APd0.2Cd1.5IF-8@AAC catalyst (i): contact angle of B–Pd0.2Cd1.5IF-8 catalyst (j): contact angle of A-Pd0.2Cd1.5IF-8@AAC catalyst.

diffractometer with Cu kα-1radiation (λ = 0.15,406 nm). Thermogravimetric analysis (TGA) was taken with a thermogravimetric analyzer (Perkin-Elmer TGA-7, USA).The types of Pd and Cd were analyzed by X-ray photoelectron spectroscopy (XPS) by using Al Kα, the pass energy was 2 min, the operand power was 250 W, and the scan step-size was 5 eV/step. A SL200B instrument was used to measure the surface wettability of different samples.

2.4.1. Method A A solution of 1.5 mmol cadmium nitrate and different amounts of PdCl2 in 50 mL methanol and 10 mL distilled water was added into a solution of 4.5 mmol 2-methylimidazole in 40 mL methanol, producing a brown PdxCdyIF-8 suspension. The obtained PdxCdyIF-8 suspension was mixed with the AAC and treated with ultrasonic vibration at room temperature for 2 h. Then the mixture was transferred into hydrothermal reactor and maintained at 180 °C for 12 h. After that, the pure APdxCdyIF-8@AAC nanocrystals (x = 0 y = 1.5, x = 0.1 y = 1.5, x = 0.2 y = 1.5 and x = 0.3 y = 1.5) were collected by repeated centrifugation and washed with methanol and distilled water and then dried under 120 °C for 8 h. Other samples were synthesized by the same procedure and the detail compositions of A-PdxCdyIF-8@AAC nanocrystals are shown in Table 1. 2.4.2. Method B A solution of 1.5 mmol cadmium nitrate and different amounts of PdCl2 (0, 0.1, 0.2, 0.3 and 0.4 mmol) in 50 mL methanol and 10 mL water was added into a solution of 4.5 mmol 2-methylimidazole in 40 mL methanol, producing a brown PdxCdyIF-8 suspension. The obtained PdxCdyIF-8 suspension was mixed with the AAC and treated with ultrasonic processing at room temperature for 2 h and then under stirring 12 h. After that, the pure B-PdxCdyIF-8@AAC nanocrystals (x = 0 y = 1.5, x = 0.1 y = 1.5, x = 0.2 y = 1.5 and x = 0.3 y = 1.5)

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Table 2 Physical properties of all samples. Sample

SBET/m2/g

Smic/m2/g

Vtot/m3/g

Vmic/m3/g

Pore size/nm

Pd loading/wt%

CdIF-8@AAC A-Pd02Cd1.5IF-8@AAC B–Pd0.1Cd1.5IF-8@AAC B–Pd0.2Cd1.5IF-8@AAC B–Pd0.3Cd1.5IF-8@AAC

1503.0 829.5 1371.8 1282.3 1095.1

1092.6 568.3 942.2 823.1 786.9

0.8225 0.2268 0.6853 0.5726 0.4869

0.5361 0.2038 0.4852 0.4237 0.3822

0.685 0.586 0.672 0.668 0.622

– 1.3659 0.6876 1.3659 2.0349

SBET: the specific surface area; Smic: the micropore surface area; Vtot: total pore volume; Vmic: micropore volue

were collected by repeated centrifugation and washed with methanol and water and dried under 120 °C for 8 h. Other samples were synthesized by the same procedure and the detail compositions of BPdxCdyIF-8@AAC nanocrystals are also shown in Table 1. 2.5. Fabrication of H2O2 sensor Prior to modification, the bare GCE was polished to a mirror-like surface using 0.05 μm and 0.3 μm Al2O3 power on the tracing paper. After that, the GCE was sonicated with nitric acid (1:1), ethyl alcohol and deionized water, respectively and dried by nitrogen gas. 5 μL (1.0 mg mL−1) of A(B)-PdxCdyIF-8@AAC suspension was carefully drop cast onto the freshly polished GCE and dried under infrared light to form A(B)-PdxCdyIF-8@AAC modified GCE. The electrodes were stored in a refrigerator at 4 °C under dry conditions when not in use. 2.6. Fabrication of the A(B)-PdxCdyIF-8@AAC//AAC asymmetric capacitor The asymmetric supercapacitors were assembled with the A(B)PdxCdyIF-8@AAC composites as the positive electrode, activated carbon as the negative electrode and polypropylene diaphragm paper as the separator. The electrochemical measurements of the fabricated A(B)NixMoy-MOFs@AAC//activated carbon asymmetric supercapacitor were implemented in 7 M KOH aqueous electrolyte in a two-electrode cell at room temperature.

3. Results and discussion 3.1. Characterization of PdxCdyIF-8@AAC nanocomposites The morphologies and microstructure of A(B)-PdxCdyIF-8@AAC nanocomposites were researched by the field emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM). Fig. 1(a–c) show the FESEM images of A-CdIF-8@ AAC nanocomposites. Unlike other nanocomposites, the A-CdIF-8 displays a relatively smooth surface and is randomly anchored on the surface of AAC. Fig. 1(d–i) show the FESEM images of A-Pd0.2Cd1.5IF-8@AAC, the original cubic Pd nanoparticles with the size of 3–5 nm are closely anchored on the suface of the CdIF-8 and aggregated to some extent which may impede their catalytic properties. CdIF-8 nanoparticles are globular in shape with an average diameter of 15 nm, which are loaded on the surface (Fig. 1(d–f)) and embedded inside of the AAC (Fig. 1(g–i)). Such results well match with the TEM images in Fig. 1(p–r). Interestingly, as shown in Fig. 1(j–o), CdyIF-8 with the hexahedron shape rather than globular shape, were evenly dispersed on the AAC without any aggregation. The original cubic Pd nanoparticles with the size of 3–5 nm are dispersed on the surface of the CdIF-8, and CdIF-8 with the size of 10 nm are loaded on the surface (Fig. 1(j–l)) and embedded inside of the AAC (Fig. 1(m–o)) which could also be confirmed from TEM results ((Fig. 1(s–u)). Furthermore, the hydrophilicity of the A-Pd0.2Cd1.5IF-8, APd0.2Cd1.5IF-8@AAC, B–Pd0.2Cd1.5IF-8 and B–Pd0.2Cd1.5IF-8@AAC hybrids were determined by the contact angle of deionized water and exhibited in Fig. 2(g–j). Comparing with the A-Pd0.2Cd1.5IF-8, the contact angle of water on the A-Pd0.2Cd1.5IF-8@AAC is reduced from 124.5° to 92.4°. The contact angle of water on the B–Pd0.2Cd1.5IF-8@AAC hybrids is reduced to 76.8°. The hydrophilic characteristics of B–Pd0.2Cd1.5IF-8@AAC hybrids greatly increase the contactable area between electrolyte and electrode, which are conducive to improving the kinematic velocity of ions through the surface of the active materials. Thus, the effective utilization of electroactive materials can be further improved[29]. The permanent porosity of the obtained A(B)-PdxCdyIF-8@AAC catalysts were confirmed by the nitrogen-sorption measurements at 77 K and showed in Fig. 2(a and b), and the calculated BET surface areas were listed in Table 2. The N2 adsorption isotherms of the A(B)-PdxCdyIF-8@AAC catalysts exhibits the type І isotherms as classified by

Fig. 3. (a) Cyclic voltammograms of B–Pd0.2Cd1.5IF-8@AAC electrodes by feeding with O2, N2 and air (b) cyclic voltammograms of B–Pd0.2Cd1.5IF-8@AAC electrodes in different pH solution (c) the cyclic voltammograms of different modified electrodes.

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Fig. 4. (a) Cyclic voltammograms with different scanning rates of A-Cd1.5IF-8@AAC (b) cyclic voltammograms with different scanning rates of A-Pd0.2Cd1.5IF-8@AAC (c) cyclic voltammograms with different scanning rates of B–Pd0.2Cd1.5IF-8@AAC (d) the analog line of peak currents and the square root of scan rates of A-Cd1.5IF-8@AAC and A(B)–Pd0.2Cd1.5IF8@AAC (e) electrochemical stability of A(B)–Pd0.2Cd1.5IF-8@AAC (f) Nyquist plots of A(B)-PdxCdyIF-8@AAC

IUPAC, the steep increases appearing at the very low relative pressure (b0.1) in N2 uptake exhibits the characteristic of microporous hybrids. The existence of the textural meso/ macro-porosity could also be noted at high relative pressure of N2 uptake. The microporous structure of A-PdxCdyIF-8@AAC has an inner cage of 1.1–1.2 nm and 1.5 nm. Unlike the pore size distribution of A-PdxCdyIF-8@AAC, the microporous structure of APdxCdyIF-8@AAC has a relatively low inner cage at about 0.6 nm and 1.25 nm, which should be due to the aggregation of Pd nanoparticles and the destruction of pore the during the reaction, especially upon heating that may impede the catalytic properties. As shown in Table 2, comparing with the specific surface area CdIF-8@AAC (1503.0 m2/g), the specific surface areas of A-Pd02Cd1.5IF-8@AAC and B–Pd02Cd1.5IF-8@AAC are 829.5 and 1282.3 m2/g, respectively. While the micropore volumes are 0.2038 cm3/g and 0.4237 cm3/g, respectively. Some differences could be observed in the sorption isotherms and surface area for the obtained A(B)-PdxCdyIF-8@AAC catalysts, which may be due to the difference of Pd distribution. The addition of Pd may block up the pores and reduce the specific surface area of A-Pd02Cd1.5IF-8@AAC and B–Pd02Cd1.5IF-8@AAC. The loading of the Pd nanoparticles could not alter the pore size distribution of A(B)-PdxCdyIF-8@ AAC catalysts, which well match with the facts that the Pd nanoparticles are adsorbed on the forming fresh surfaces of the growing CdIF-8 and too large to occupy the cavities of A(B)-PdxCdyIF-8@AAC catalysts. On the other hand, the Pd nanoparticles are usually not uniformly dispersed and readily aggregate to some extent during the reaction and the collapse of the pore[30], especially upon heating. Such results are consistent with the tests of Fig. 2(a and b). The thermal stabilities of the A(B)-PdxCdyIF-8@AAC nanohybrids were also determined by thermogravimetric analysis (TGA) and the corresponding images were exhibited in Fig. 2(c). The major weight loss (9.2%) of A-PdxCdyIF-8@AAC occurred in two stages. The first weight loss existed around about 70–100 °C, which ascribed to the evaporation of physically adsorbed water. The second one (36.8%) major occurred between 300 and 600 °C, which represented the pyrogenic decomposition of CdyIF-8 and the loss of functional groups on AAC. However, the major weight loss (17.2%) of B-PdxCdyIF-8@

AAC occurred around 400 °C, which contributed to the pyrogenic decomposition of CdyIF-8. Such results indicated that the B-PdxCdyIF-8@AAC had a better thermal stability. Meanwhile, the crystalline nature of the A(B)-PdxCdyIF-8@AAC was determined by XRD method and exhibited in Fig. 2(d). Some sharp diffraction peaks of PdxCdyIF-8@AAC with different molar ratios of Pd/Cd2+ appeared which were the characteristic of the CdIF-8 crystals, suggesting that the obtained PdxCdyIF-8@AAC own the same crystal structure as the pristine CdIF-8. Beyond that, a characteristic diffraction peak at approximately 40° can be observed, which could be indexed to the (111) plane of the face-centered cubic Pd. These confirm the successful immobilization of palladium nanoparticles within CdIF-8. And the intensity is decreased with the reduced proportion of the Pd nanoparticles in the total A(B)-PdxCdyIF-8@AAC [31,32]. XPS analysis in Fig. 2(e and f) exhibited the surface species and electronic performances of A-Pd0.2Cd1.5IF-8@AAC and B–Pd0.2Cd1.5IF-8@AAC hybrids. Fig. 2(e) shows the binding energies of Cd 3d5/2 and Cd 3d3/2 for the A(B)–Pd0.2Cd1.5IF-8@AAC and A(B)– Pd0.3Cd1.5IF-8@AAC are distributed at 404.2, 412.1 eV, 405.3 and 414.1 eV, respectively. The separation distance between two peaks is 6.8 eV, indicating that Cd atoms are in the complete A(B)-PdXCdYIF-8@AAC phase. The Pd 3d region of the XPS spectra of APd0.2Cd1.5IF-8@AAC and A-Pd0.3Cd1.5IF-8@AAC are exhibited in Fig. 2(f). The APd0.2Cd1.5IF-8@AAC and A-Pd0.3Cd1.5IF-8@AAC shows Pd 3d3/2 and Pd 3d5/2 characteristic peaks with binding energies are distributed at about 340.6 and 335.1 eV, respectively, which demonstrate the existence of metallic Pd0 anchored on the AAC. The other pair of Pd signals occurring at 336.7 and 342.3 eV was ascribed to Pd atoms with lower charge density (Pd2+). However, the B–Pd0.2Cd1.5IF-8@AAC and B–Pd0.3Cd1.5IF-8@AAC presents peaks characteristic of Pd0 3d3/2, Pd0 3d5/2, Pd2+ 3d3/2 and Pd2+ 3d5/2 at about 339.0, 343.6, 338.7 and 345.2 eV, respectively. The binding energies of Pd 3d and Cd 3d in APd0.2Cd1.5IF-8@AAC and A-Pd0.3Cd1.5IF-8@AAC catalyst shifted to lower energy compared with that in B–Pd0.2Cd1.5IF-8@AAC and B–Pd0.3Cd1.5IF-8@AAC catalysts [33–35]. It demonstrated that the Pd–CdIF-8 miscible phase was partially formed that caused the unique surface electronic structural, which well match with the result of XRD.

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Fig. 5. (a) DPV curves of A-Cd1.5IF-8@AAC with different concentrations (b) DPV curves of A-Pd0.2Cd1.5IF-8@AAC with different concentrations (c) DPV curves of B–Pd0.2Cd1.5IF-8@AAC with different concentrations (d) the analog line of peak currents and the concentration of A-Cd1.5IF-8@AAC and A(B)–Pd0.2Cd1.5IF-8@AAC.

3.2. Electrochemical H2O2 sensing behavior Cyclic voltammetry is a commonly used method for the electrochemical evaluation of the modified electrodes. A-Pd0.2Cd1.5IF-8@AAC sensor and B–Pd0.2Cd1.5IF-8@AAC sensor were selected for the amperometric determination of H2O2. In order to create an appropriate testing environment, our groups have also researched on the effects of the pH and the feeding gases. As shown in Fig. 3(a), the reductive peak current with A-Pd0.2Cd1.5IF-8@AAC catalyst is 0.212 mA and 0.206 mA upon feeding with O2 and air, respectively, which are both higher than that upon feeding with N2 (1.16 mA). This might ascribe to the enhancement of the reduction of O2 when dissolved oxygen easily diffuses to the surface of electrode. There is no obvious difference in the reductive peak current with A(B)– Pd0.2Cd1.5IF-8@AAC upon feeding O2 and air. Thus, the A(B)-PdxCdyIF-8@AAC sensors upon feeding air were selected. As shown in Fig. 3(b), the beginning of reduction shifts to positive potential at the pH of 9.0, which clearly stated that the H2O2 is reduced more easily in the slight alkalinity and pH = 9.0 of electrolyte was selected for the amperometric determination of H2O2. To further study the reaction kinetics about H2O2 toward A(B)–Pd(0)0.2Cd1.5IF-8@AAC modified GCE, the reaction kinetics about H2O2 at CdIF-8@AAC/GCE was also examined for comparisons. The electrocatalytic activities of A-CdIF-8@AAC, A-PdxCdyIF-8@AAC and BPdxCdyIF-8@AAC electrodes for H2O2 oxidation were probed by cyclic voltammetry (CV) between −0.6 and 0.4 V versus in the phosphate buffer solution (PBS, pH = 9 0.1 M) containing 2 μM H2O2 at a 100 mV s−1 scan rate. As can be seen in Fig. 3(c), a weak reduction peak can be observed at about −0.3 V with a corresponding oxidation peak on A(B)–CdIF8@AAC, which suggested weak catalytic activity in the applied potential range. Such result may be ascribed to the formation of some residual functional groups in A(B)–CdIF-8@AAC. When compared to the reduction current of A-Pd0.2Cd1.5IF-8@AAC electrodes, the reduction current of B–Pd0.2Cd1.5IF-8@AAC electrodes increased by 10%, which is contributed to coordination of B–Pd0.2Cd1.5IF-8 and AAC. It could be obviously noticed that the electrochemical reduction peak current of H2O2 on B-PdxCdyIF-8@AAC electrode increased at −0.2 V and considerably higher than on A-PdxCdyIF-8@AAC electrodes, which proved that the excellent synergistic effect between Pd and CdIF-8 nanoparticles. It could be clearly observed that the A-Pd0.2Cd1.5IF-8@AAC sensor exhibited the best sensitivity among the A-PdxCdyIF-8@AAC sensors. The B–Pd0.2Cd1.5IF-8@AAC sensor showed the best sensitivity among the B-PdxCdyIF-8@AAC sensors. The phenomenon could be attributed to the distribution of Pd. Pd as the main catalytic agent, could effectively detect

H2O2. When the mole ratio of Pd and Cd is 0.2:1.5, Pd could be evenly distributed at the surface of CdIF-8. Comparing with B–Pd0.2Cd1.5IF-8@AAC, A-Pd0.2Cd1.5IF-8@AAC was synthetized by violently hydrothermal solvothermal method and oxygen-containing or other functional groups of CdIF-8 and AAC were reduced. Therefore, Pd+(Pd2+) could not anchor firmly on the surface of CdIF-8 and AAC. Besides, the particle size of APd0.2Cd1.5IF-8 is larger than that of B–Pd0.2Cd1.5IF-8, the relative electrochemical reaction only takes place on its surface in the initial cycles [36–38]. Thus, it could be obviously noticed that the anodic and cathodic peak current linearly increased with the increase of the square root of scan rate, suggesting a typical diffusion-controlled. It also could be observed that the electrochemical reduction peak potential of H2O2 was slightly shifted to negative potential with the increase of the scan rate as shown in Fig. 4(a–c), which demonstrated the characteristics of an irreversible electrochemical reduction reaction. In addition, preeminent linear relationship is calculated between the square root of scan rate and reduction peak current of H2O2 and exhibited in Fig. 4(d). The electroactive surface area of A(B)– Pd(0)0.2Cd1.5IF-8@AAC and CdIF-8@AAC electrodes were tested using CVs in PBS(pH = 9.0 0.1 M). In the electrochemical reaction, the peak current (Ip) is relative to the square root of the scan rate (v1/2) as shown in the following equations [2,17]:   1=2 IPa ¼ 2:69  105 n3=2 AD0 C 0 v1=2

ð1Þ

Ipa ¼ kv1=2

ð2Þ

Where v is the scan rate, C0 is the concentration of PBS, D0 is 6.56 × 10−6 cm2 s−1, A is the surface area of the bare electrode, Ipa is the peak current, k is the fitting line slope of Ipa vs. v1/2 which is a coefficient related only to D0, expressing the mass transfer rate of the samples. The value increased with the increase of the mass transfer. The calculated values for k were in the following order: B–Pd0.2Cd1.5IF-8@AAC(0.4629)NA-Pd0.2Cd1.5IF-8@AAC (0.3871)NCd1.5IF-8@AAC(0.2276), which demonstrated the B–Pd0.2Cd1.5IF-8@AAC could enhance the mass transfer rate. The surfaces of these catalysts provide the reaction site and the location for the supply and reception of electrons. Especially, the B–Pd0.2Cd1.5IF8@AAC features highly abundant Pd. The CdIF-8 anchoring on AAC may have a significant activation effect on the molecules/ions which participate in the electrochemical reaction. Besides the excellent electrocatalytic properties, the high electrochemical stability has also played a significant role in the practical applications of electrochemical sensors. To

8

Y. Li et al. / Materials and Design 186 (2020) 108267

Table 3 The properties of the various MOF of other hybrids modified electrodes for detection of H2O2. Electrode

Electrolyte

Sensitivity

Detection limit (μM) (S/N = 3)

Linear range (μM)

Ref.

Cu-TDPAT-n-ERGO sensor Au/ZnO-PVA/CAT/Chitosan bio-electrode Nf/Pd@Ag/rGO-NH2/GC sensor Au@ZIF-67 NS-rGO acid-pPdAu/GO 3DOM-SmCoO3 CF@NCNTAs–GNPs MIL-53-CrIII A-Pd0.2Cd1.5IF-8@AAC

0.1 M PBS (pH 7.0)

∼ 210.49 μAμM−1cm−2 1309.06 μAμM−1cm−2 7553 μAμM−1cm−2 ∼ ∼ 460 μAμM−1cm−2 142 μAμM−1cm−2 11.9 μAμM−1 0.266 μAμM−1

0.17 9.13 nM 0.7 19 nM 0.45 μM 0.1 nM 0.004 μM 50 nM 3.52 0.016

4–1200 1–17 0.002–19.500 0.1–3.0 mM 7–18,000 μM 0.3–1.0 mM 0.1–10000 ∼ 25–500 μM 2 μM to 17.5 mM

[17] [10] [42] [2] [5] [18] [14] [43] [1] In this work

0.1 M PBS (pH 7) 0.1 M PBS (pH 9.0) 0.2 M PBS (pH 7) PBS, pH 9.0 0.1 M NaOH solution 0.1 M PBS (pH 9.0) 0.1 M NaOH solution 0.1 PBS (pH = 9.0)

effectively investigate the long-term electrochemical stability of A(B)–Pd0.2Cd1.5IF-8@ AAC, accelerated durability tests were carried out by applying cyclic potential scan at 0.1 V s−1 in 0.1 M PBS (pH = 9) solution. The electrochemical stability was examined by the integration of the redox peak of A(B)–Pd0.2Cd1.5IF-8@AAC before and after the accelerated durability tests. Fig. 4 (e) exhibits the first and the 100th voltammetric curve obtained on A(B)–Pd0.2Cd1.5IF-8@AAC. A-Pd0.2Cd1.5IF-8@AAC lost 10.6% of the integrated peak area after 100 voltammetric cycles. However, the B–Pd0.2Cd1.5IF-8@AAC experienced a loss of only 2.4% after 100 cycles. Such results strongly indicate that B–Pd0.2Cd1.5IF-8@ AAC could effectively improve the stability of the sensor material. Thus, the BPd0.2Cd1.5IF-8@AAC sensor is more suitable to be used as a catalyst for the detection of hydrogen peroxide [39–41]. EIS measurements were performed in 0.1 M PBS (pH = 9) solution to further investigate the characteristics of ion and charge transfer in A(B)–Pd0.2Cd1.5IF-8@AAC based electrodes. Fig. 4 (f) exhibits the Nyquist plots of the CdIF-8@AAC and A(B)–Pd0.2Cd1.5IF-8@ AAC based electrodes with high frequency region in the inset. The fitted equivalent circuit contains charge transfer resistance (Rct) and solution resistance (Rs). Obviously, the diameter of the semicircle for CdIF-8@AAC is much larger than that of A(B)–Pd0.2Cd1.5IF-8@ AAC, while the diameter of the semicircle for B–Pd0.2Cd1.5IF-8@AAC is a little smaller than that of A-Pd0.2Cd1.5IF-8@AAC in the high frequency region, which ascribes to the particle size of A-Pd0.2Cd1.5IF-8 is large than B–Pd0.2Cd1.5IF-8. And thus it is relatively difficult

for A-Pd0.2Cd1.5IF-8 to have shorter ion diffusion depth, more reactive sites and larger specific surface area. The inclination at 45° in the middle frequency range is related to limited ionic diffusion and represented by Warburg resistance. B–Pd0.2Cd1.5IF-8 with negligible Warburg region shows lower Rs and Rct than those of A-Pd0.2Cd1.5IF-8, inferring efficient accessibility of the electrolyte ions to the electrode. As a promising method to scrutinize the electrochemical activity and calculate the electrochemical parameters of the modified electrodes, The DPV technique is adopted to evaluate the limit of detection and linear range and sensitivity of target analyte under the optimized conditions. As shown in Fig. 5(a–c). It could be easily observed that the cathodic currents increased with the increase content of H2O2, suggesting the excellent electrocatalytic activity of the A(B)–Pd0.2Cd1.5IF-8@AAC toward H2O2 reduction. Besides, the reduction of H2O2 began at the same potential of the reduction of Pd (Ⅱ), and the finding demonstrated the electrocatalytic reduction of H2O2 occurred by the center metallic cation of A(B)–Pd0.2Cd1.5IF-8@AAC. Therefore, the corresponding electrocatalytic mechanism could be proposed as follows: PdðⅡÞ þ e− →PdðІÞ

ð3Þ

PdðⅠ Þ þ e− →Pdð0Þ

ð4Þ

Fig. 6. Electrochemical property of A(B)-PdxCdyIF-8@AAC//AC: (a) The GCD curves of A(B)-PdxCdyIF-8@AAC//AC with various mass ratios at the current density of 0.5 A g−1 (b) the GCD curves of B–Pd0.2Cd1.5IF-8@AAC//AC at various current densities. (c) CV curves of A(B)-PdxCdyIF-8@AAC//AC with various mass ratios at the scan rate of 0.1 V s−1 (d) CV curves of the B– Pd0.2Cd1.5IF-8@AAC//AC at various scan rates.

Y. Li et al. / Materials and Design 186 (2020) 108267

9

Fig. 7. Electrochemical property of A(B)-PdxCdyIF-8@AAC//AC (a) specific capacitance of A(B)-PdxCdyIF-8@AAC//AC at different current densities (b) Nyquist plots of A(B)-PdxCdyIF-8@ AAC//AC electrodes in 7 M KOH in the frequency range from 10−2 to 105 Hz (c) cycling property of A(B)–Pd0.2Cd1.5IF-8@AAC//AC at the current density of 5 A g−1. Inset: The corresponding GCD curves of the first and the last 12 cycles of the asymmetric supercapacitor during 10,000 GCD cycles. (d) Ragone plot for energy density and power density.

2H 2 O2 →2O2 þ 4Hþ þ 4e−

ð5Þ

O2 þ 4Hþ þ 4e− →2H 2 O

ð6Þ

2H 2 O2 →O2 þ 2H2 O þ 2e−

ð7Þ

Firstly, the H2O2 could be recognized and absorbed to the surfaces and pores of A(B)– Pd0.2Cd1.5IF-8@AAC. Then in the cathodic pathway A(B)–Pd ðⅡÞ 0.2Cd1.5IF-8@AAC was

reduced electrochemically to A(B)–Pd(0)0.2Cd1.5IF-8@AAC, and the H2O2 reduction was catalyzed by A(B)–Pd(0)0.2Cd1.5IF-8@AAC. Simultaneously, the catalytic center Pd (II) was returned to Pd (0) by H2O2. The oxidation signals of A-Pd0.2Cd1.5IF-8@AAC, B– Pd0.2Cd1.5IF-8@AAC and Cd1.5IF-8@AAC (IP, mA) were proportional to their concentrations (C, mM) from 2 μM to 17.5 mM, 10 μM to 10.2 mM and 50 μM to 4.75 mM, respectively. As shown in Fig. 5(d), the linear regression equations are expressed as IP1(mA) = 0.2659x+0.0013,557 (R2 = 0.9981) (sensitivity, 0.266 μA/μM), IP2(mA) = 0.1953x+0.002531 (R2 = 0.9892) (sensitivity, 0.195 μA/μM) and IP3(mA) =

Table 4 Comparison of obtained results with reported similar systems. Composite

Electrolyte

Potential Specific capacitance/F g−1 window/V

CoNi-MOF

1 M KOH

0.35 V

CuO/MOF

2 M KOH

1.8 V

Zn-doped Ni-MOF

0.35 M H2SO4

1.2 V

Energy density/Wh kg−1

Power density/W kg−1

Cyclic stability

Ref.

1104 at 2 A g−1

28.5

15,000

∼

13.6 F cm2 at 2 mA cm2

1.7 mW h cm2 27.56

4.0 mW cm2

69.7% up to 2000

1750

66% up to 3000

3.14 mWh cm−3 21.05

1.26 mWh cm−3 60,300

90% after 2000 cycles

–

–

–

25.4

400

78.85

493

93% capacitance retention after 10,000 cycles. 84.38% up to 10,000

Ref [44] Ref [45] Ref [46] Ref [47] Ref [48] Ref [49] Ref [50]

854 F g

−1

at 10 A g

−1

2

Cu-based MOF HKUST-1 Ni-MOF

0.5 M Na2SO4

1.8 V

0.3 M KCl

0.9 V

1812 mF/cm at current densities of 1 mA/cm 87 F g−1 at 0.5 A/g

Zr-MOF

6 mol dm−3 KOH 1 M KOH

0.75 V

1144 F g−1 at scan rate of 5 mV s−1

0.5 V

996 F g

7 M KOH

1.8 V

816 at 0.5 A g

Zn-based metal-organic frameworks This work

−1

at 1.0 A g

−1

70% after 2500 cycles

10

Y. Li et al. / Materials and Design 186 (2020) 108267

0.05541x+0.0003827 (R2 = 0.9923) (sensitivity, 0.055 μA/μM), respectively. The detection limit is evaluated to be 0.016, 0.028 and 0.083 μM based on 3 signal-noise ratios, respectively. It could be obviously observed that the responses to H2O2 are comparable to the reported non-enzymatic H2O2 sensors based on different MOFs hybrids or other nanocomposites and the corresponding results were exhibited in Table 3.

basis of the total mass of active material. Nevertheless, the power density has raised from 0.435 kW kg−1 to 23.386 kW kg−1 and from 0.563 kW kg−1 to 37.579 kW kg-1 at the current density ranging from 1 to 25 A g−1. The excellent electrochemical performance for energy storage could be comparable to reported similar systems, as shown in Table 4.

4. Conclusions 3.3. Electrochemical capacitive performance The capacitive performances of asymmetric supercapacitors with A(B)-PdxCdyIF-8@ AAC as positive electrode and porous activated carbon as negative electrode (the average mass of A(B)-PdxCdyIF-8@AAC//AAC is 0.0054 g) were tested in a two-electrode configuration with 7 M KOH electrolyte and performed in Fig. 6 and Fig. 7. Fig. 6 (a) showed the galvanostatic charge-discharge (GCD) curves of asymmetric supercapacitors at the current density of 0.05 A g−1 with different mass ratios. The GCD curves had no overcharge when the voltage increased to 2 V and showed nonlinear profile, demonstrating the specific capacitance of asymmetric supercapacitors was mainly attributed to pseudocapacitive charge storage along with enhanced typical EDLC characteristics. The charge storage mechanism of Pd and Cd is based on adsorption or intercalation process on the surface and in bulk respectively. þ

Pd→Pd þ e− þ

Pd →Pd

2þ

2þ

Cd→Cd

2þ

Cd þ Pd

þe

ð8Þ −

ð9Þ

−

ð10Þ

þ 2e

2þ

→Cd

þ Pd

In conclusion, we have developed A(B)-PdxCdyIF-8@AAC metalorganic framework (MOF). Superior conductivity of activated carbon provides an ease to electrochemical reactions enhancing the electrochemical property of A(B)-PdxCdyIF-8@AAC as supercapacitor and nonenzymatic ultrasensitive H2O2 detection. A(B)-PdxCdyIF-8@AAC was fabricated with different appearances in different synthetic methods. In comparison to other modified electrodes, the B–Pd0.2Cd1.5IF-8@AAC showed higher sensitivity, lower detection limit and wider linear ranges in the presence of interfering species for H2O2 owing to its faster diffusion, faster electron transfer kinetics and large surface area. The results demonstrated that the B–Pd0.2Cd1.5IF-8@AAC hybrids could replace noble metal materials for accurate determination H2O2. Additionally, the B–Pd0.2Cd1.5IF-8@AAC hybrid is desired to be applied as electrode materials for asymmetric supercapacitor, even for applications beyond supercapacitor.

ð11Þ

Besides, the specific capacitance of A-Pd(0)0.2Cd1.5IF-8@AAC//AAC has achieved 452 F g−1 It is noteworthy that the B–Pd(0)0.2Cd1.5IF-8@AAC//AAC supercapacitor shows the longest charge/discharge time, demonstrating the highest specific capacitance (502 F g−1). Fig. 6(b) describes the GCD curves of B–Pd(0)0.2Cd1.5IF-8@AAC//AACs at various current densities. The curves displayed a nearly symmetry charge/discharge curves with negligible voltage drop, indicating a good capacitive behavior and small internal resistance of the electrode. As shown in Fig. 7(a), the B–Pd(0)0.2Cd1.5IF-8@AAC//AAC has achieved the eminent specific capacitance of 502, 459, 422, 395, 370, 361, 346 and 342 F g−1 at the current densities of 0.05, 0.1, 0.5, 1,5,10, 20 and 25 A g−1, respectively. With the increase of current density, the specific capacitance decreases slowly, demonstrating the electrode allows the swift ion diffusion. In addition, the transport characteristics of charge carriers and the interface ion absorption-desorption rates were examined by electrochemical impedance spectroscopy (EIS) measurements in the 7.0 M KOH over the frequency range from 0.01 Hz to 100 KHz and summarized in Fig. 7(b). Obviously, all the Nyquist plots include an observed straight line in the low frequency region which could respond to the diffusion-controlled Warburg impedance, and a similar semicircle in high-frequency region. It could be observed that the semicircle in the high-frequency range for B-PdxCdyIF-8@AAC electrodes is smaller than that of A-PdxCdyIF-8@AAC electrodes, demonstrating B-PdxCdyIF-8@AAC electrodes have a smaller resistance and fast/ easy transfer of electrons/ions to the inlayer of the material. The result demonstrated that B-PdxCdyIF-8@AAC electrodes are favorable for electrolyte to increase the kinematic velocity of ions through the surface of the active materials, thus increasing the contactable area between the electrolyte and electrode. The cycling stability of the A(B)–Pd(0)0.2Cd1.5IF-8@AAC//AACs was further investigated by repeated charge/discharge test at the current density of 10 A g−1 for 10,000 cycles, and the result summarized in Fig. 7(c). Noticeably, after 10,000 charge/discharge cycles the specific capacitance of A-Pd(0)0.2Cd1.5IF-8@AAC//AAC and B–Pd(0)0.2Cd1.5IF8@AAC//AAC could respectively maintain 78.313% and 83.579% of their initial values, which are much better than that of the ASC reported previous, demonstrating the A(B)– Pd(0)0.2Cd1.5IF-8@AAC//AACs are remarkable for long time electrochemical stability. Fig. 6(c) exhibits the CV curves of A(B)–Pd(0)xCdyIF-8@AAC//AACs at 50 mV s−1 in the potential windows of 0–1.5 V. It is noted that all curves of B–Pd(0)xCdyIF-8@AAC//AACs contain a palpable broad redox peaks attributed to the faradaic redox reactions, demonstrating the pseudocapacitive characteristic. This arises from the reversible faradaic reaction of PdCdIF-8 in alkaline electrolyte. The peaks for PdCdIF-8 are centered on 0.6 V, proving the mutual transformation between Pd, Pd+ and Pd2+. And the redox peaks shifted to more negative regions with the increase of mole ratio of Pd, which are typical for the transformation between Pd, Pd+ and Pd2+. Such results are in good agreement with our XPS analysis. As for P//AACs, peak intensity and peak separation are lower than that of B–Pd(0)xCdyIF-8@AAC//AACs, demonstrating the B–Pd(0)xCdyIF-8@AAC//AACs have a better reaction activity and reversibility. Furthermore, the B–Pd(0)0.2Cd1.5IF-8@ AAC//AACs exhibits the largest CV area, which in agreement with the result of GCD curves. Fig. 6(d) shows the homologous CV curves of B–Pd(0)0.2Cd1.5IF-8@AAC//AAC at various scan rates. Evidently, with the increase of scan rate, the CV area becomes larger and the spread-dominated devotion becomes weaker, leading to the reduction of specific capacitance at high scan rates. Furthermore, the peaks transfer slightly along with the scan rate, which may attribute to the reaction capability and ion/electron transfer at the interface of electrode and electrolyte. The Ragone curve of A(B)–Pd(0)0.2Cd1.5IF-8@AAC//AACs for energy density (E, Wh kg−1) and power density (P, kW kg−1) in a voltage window of 0–2 V at different current densities are illustrated in Fig. 7(d). The specific energy density of A-Pd(0)0.2Cd1.5IF-8@AAC//AAC and B–Pd(0)0.2Cd1.5IF-8@AAC//AAC decreased from 67.67 Wh kg−1 to 35.27 Wh kg−1 and from 83.847 Wh kg−1 to 51.955 Wh kg−1 on the

Declaration of interest statement All persons who meet authorship criteria are listed as authors, and all authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in any other publication before its appearance in “Materials & Design”. CRediT authorship contribution statement Yue Li: Data curation, Formal analysis, Investigation, Methodology, Supervision, Writing - original draft. Lanshu Xu: Supervision, Writing - review & editing. MengYing Jia: Conceptualization. Linlin Cui: Conceptualization. Xinyan Liu: Writing - review & editing. XiaoJuan Jin: Writing - review & editing. Acknowledgements The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (31470605). References [1] N.S. Lopa, M.M. Rahman, F. Ahmed, S. Chandra Sutradhar, T. Ryu, W. Kim, A basestable metal-organic framework for sensitive and non-enzymatic electrochemical detection of hydrogen peroxide, Electrochim. Acta 274 (2018) 49–56. [2] H. Dai, Y. Chen, X. Niu, C. Pan, H. Chen, X. Chen, High-performance electrochemical biosensor for nonenzymatic H2O2 sensing based on Au@C-Co3O4 heterostructures, Biosens. Bioelectron. 118 (2018) 36–43. [3] M. Wang, X. Kan, Multilayer sensing platform: gold nanoparticles/prussian blue decorated graphite paper for NADH and H2O2 detection, Analyst 143 (2018) 5278–5284. [4] D. Valera, M. Sánchez, J.R. Domínguez, J. Alvarado, P.J. Espinoza-Montero, P. Carrera, et al., Electrochemical determination of lead in human blood serum and urine by anodic stripping voltammetry using glassy carbon electrodes covered with Ag–Hg and Ag–Bi bimetallic nanoparticles, Anal. Method. 10 (2018) 4114–4121. [5] T. Zhang, C. Li, Y. Gu, X. Yan, B. Zheng, Y. Li, et al., Fabrication of novel metal-free "graphene alloy" for the highly efficient electrocatalytic reduction of H2O2, Talanta 165 (2017) 143–151. [6] H. Zhao, Y. Chen, Q. Peng, Q. Wang, G. Zhao, Catalytic activity of MOF(2Fe/Co)/carbon aerogel for improving H 2 O 2 andOH generation in solar photo–electro–Fenton process, Appl. Catal. B Environ. 203 (2017) 127–137. [7] C. Wang, H. Zhang, C. Feng, S. Gao, N. Shang, Z. Wang, Multifunctional Pd@MOF core–shell nanocomposite as highly active catalyst for p-nitrophenol reduction, Catal. Commun. 72 (2015) 29–32.

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