Synthesis of palladium–carbon nanotube–metal organic framework composite and its application as electrocatalyst for hydrogen production
There are very rare reports on using metal–organic framework (MOF) catalysts for electrochemical hydrogen production. In this study, a composite of palladium, single-walled carbon nanotube (SWCNT) and MOF-199 (Pd/SWCNTs@MOF-199) was synthesized by hydrothermal method, and its application as electrocatalyst in carbon paste electrode (CPE) structure for hydrogen production was studied. Scanning electron microscopy, X-ray diffraction, Brunauer–Emmett–Teller and thermal gravimetric analysis were used to characterize Pd/SWCNTs@MOF-199 catalyst. The performance of the proposed modified CPE for electrochemical hydrogen production was studied by cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy and chronoamperometry techniques. The effect of solution pH and the amount of binder and catalyst in the paste composition were investigated. The results showed that the CPE modified with Pd/SWCNTs@MOF-199 reveals better catalytic characteristics such as highest catalytic activity and lowest onset potential compared to CPE and CPE modified with MOF-199 for hydrogen production in aqueous solution.
KeywordsMetal organic framework composite Electrochemical hydrogen production Carbon paste electrode Electrocatalyst
Today, fossil fuels with disadvantages such as limited resources, warming up the earth and the environmental pollution are known as the main energy sources. Many efforts are being made to use other energy sources instead of fossil fuels which do not have disadvantages mentioned above. Nuclear energy, which its sources are limited, and working with it requires training of skilled manpower and use of advanced systems for protection against radioactive waste, will not be able to supply the required energy of the world.
Nowadays, hydrogen as a raw material is used in different industries . Hydrogen produces high amount of energy with almost no pollution. It is a renewable energy carrier, and its resource is infinite . An energy carrier can change energy to the forms which are usable to consumers. Although, renewable energy sources such as sun and wind cannot provide energy all the time, they could produce electric energy and hydrogen, which can be stored and transported until they are needed.
Several ways such as hydrolysis , thermal catalysis and thermochemical [4, 5], photocatalysis [6, 7], photoelectrocatalysis , steam reforming , gasification  and electrolysis [5, 11] are used to produce hydrogen. Although in the electrochemical studies platinum (Pt) is known as an excellent electrode for hydrogen generation, but because of limitation of its resource and its expensive price we should look for a suitable alternative to Pt . There are several reports which have attempted to reduce the amount of loaded Pt in the electrode body  or replace it with another catalyst [14, 15, 16].
MOFs are synthesis by linking inorganic and organic units by strong bonds that lead to combined properties of organic and inorganic porous materials [17, 18, 19]. Among the wide range of available catalysts, MOFs have recently received noticeable attention owing to their characteristics such as porosity, specific surface area and adjustable pore size [20, 21]. Related to hydrogen energy, MOFs have been mostly used for hydrogen storage [22, 23, 24, 25] and there are some reports in hydrogen generation area with photocatalytic method [26, 27, 28], and a few reports for electrochemical hydrogen generation [29, 30, 31].
In this study, a composite of MOF-199 with Pd and single-walled carbon nanotube (SWCNT) was prepared (Pd/SWCNTs@ MOF-199), and was applied for electrochemical hydrogen generation. Structure and morphology of the composite were characterized by various techniques, and its electrochemical hydrogen generation performance was compared with MOF-199 in CPE.
Apparatus and software
The electrochemical impedance measurements were performed with an Autolab potentiostat/galvanostat (PGSTAT30) equipped with FRA board, also all other electrochemical studies were done with a µ-Autolab type II potentiostat/galvanostat that the software for both of them was Nova version 1.7.8. A three electrode cell containing, Pt rod as the counter electrode, bare or modified CPE as working electrode and Ag/AgCl (3 M KCl) as the reference electrode was used. In this paper, for easier comparison with other reports all potential values are reported vs. reversible hydrogen electrode (RHE). Teflon cylinder (3.0 mm i.d.) that was firmly packed with carbon paste was used as the body of the working electrode. To creation electrical connect a stainless steel rod inserted into the carbon paste. All electrochemical experiments were done at room temperature, also before every experiment the electrolyte solution was deaerated with high purity nitrogen gas for at least 10 min.
The MOF structure was investigated between 5° ≤ 2θ ≤ 90° with X-ray diffraction (XRD) instrument model of PHILIPS 1830 with a Cu (Kα) radiation source (λ = 1.5418 Å).
The catalyst morphology and its elemental analysis was investigated with a scanning electron microscopy (SEM) (VEGA\\TESCAN-XMU) equipped with energy dispersive X-ray analysis (EDX).
Thermogravimetric analysis (TGA) was used to determine the thermal behaviour of MOF-199 and Pd/SWCNT@MOF-199 using SHIMADZU, TG-50/DTA-50. The measurements were conducted at 10 °C min−1 from room temperature.
Nitrogen adsorption–desorption isotherms were measured at 77 K by a BET instrument model of ASAP™ micromeritics 2020.
Materials and solutions
Graphite fine powder (extra pure with P.S. <50 µm), paraffin oil (spectroscopic grade, Uvasol®), H3PO4 (85 %), KCl (99.5 %), H2SO4 (98 %), Cu(NO3)2·3H2O (99.5 %), benzene-1,3,5-tricarboxylic acid (H3BTC, 95 %), PdCl2 (59 % Pd basis) and ethanol (96 %) were obtained from Merck and were used without furture purifications. The single walled carbon nanotube (>95 %) was purchased from Sigma and was treated with nitric acid (5 M) for 15 min. After treatment, the SWCNTs were filtered and washed with deionised water to remove any remained nitric acid and impurities. All aqueous solutions were prepared with deionized water.
MOF-199 was synthesized as previously reported . In brief, 2.3268 g of Cu(NO3)2·3H2O was dissolved in 24 mL deionized water, and 1.4140 g H3BTC was dissolved in 24 mL ethanol. Both solutions were mixed with magnetic stirrer and then put into a 100 mL stainless steel autoclave at 120 °C for 12 h. The product was washed with ethanol and water, and then put at 70 °C for 30 min, and after this the resultant was put at 150 °C for 30 min. The final product was MOF-199.
PdCl2 (0.16 g) was dissolved in 5 mL ethanol, 0.10 g of treated SWCNT was added to the proposed solution, and was dispersed using ultrasonic bath. After dispersion, during stirring of the mixture, 1 mL hydrazine was added dropwise under nitrogen atmosphere. Then the pH was adjusted to 9 with 1 M NaOH solution. For complete reduction of palladium the mixture was stirred on the magnetic stirrer for 2 h at 100 °C. Finally, the precipitate was filtered and washed with deionized water and then dried in oven at 100 °C for 4 h. This product is called Pd/SWCNT.
Synthesis of Pd/SWCNTs@ MOF-199 nanocomposite
Pd/SWCNTs@MOF-199 nanocomposite was prepared according to a previously reported procedure for synthesis of SWCNT@MOF-5 . For this purpose, 2.320 g of Cu(NO3)2·3H2O was dissolved in 24 mL deionized water, and 1.414 g H3BTC was dissolved in 24 mL ethanol. Both solutions were mixed with magnetic stirrer for 20 min. Then 0.100 g of Pd/SWCNTs was added to the obtained solution, and was stirred for about 24 h at room temperature. The resultant mixture was transferred to autoclave and heated at 120 °C for 12 h. The precipitate was filtered, and washed with boiling deionized water and acetone. The washed precipitate was dried in vacuum oven at 100 °C for 4 h. The final product (Pd/SWCNTs@MOF-199 nanocomposite) was characterized with BET, TGA, XRD, EDX and SEM.
Preparation of bare and modified CPEs
Paraffin oil (as binder) and graphite powder (15:85 wt%) were mixed with an agate mortar to obtain a carbon paste. The modified carbon pastes were obtained by mixing a certain amount of Pd/SWCNTs@MOF-199 or MOF-199 (as modifiers) with binder and graphite. For preparation of the bare and modified electrodes, the working electrode cavities with 3 mm diameter were filled with unmodified and modified pastes. For removing any holes and improve the conductivity the pastes were packed on a smooth surface. Before each electrochemical measurement the electrode surface was renewed, polished on a smooth weighing paper, and washed with deionized water.
Results and discussion
Morphological characterization of Pd/SWCNTs@MOF-199
X-ray diffraction (XRD) characterization of nanocomposite
Determination of nitrogen adsorption–desorption isotherms
The structure data of the pores for MOF-199 and Pd/SWCNT@MOF-199 with adsorption–desorption analysis
Vp (cm3 g−1)
d spacing (nm)
ABET (m2 g−1)
Electrochemical hydrogen evolution reaction on Pd/SWCNTs@MOF-199 modified CPE
Electrochemical studies showed that Pd/SWCNTs@ MOF-199 is a good electrocatalyst for HER. Therefore, for further study, some important parameters including electrocatalyst amounts in the electrode composition, paraffin binder and electrolyte pH were optimized.
The catalyst amount
Optimization of the paraffin binder amount
Steady-state polarization curves for HER at bare and modified CPEs
Tafel slopes (b), exchange current densities (jo) and transfer coefficients (α) for HER at CPE, MOF-199-CPE and Pd/SWCNTs@MOF-199-CPE
b (mV decade−1)
jo (mA cm−2)
6.41 × 10−6
6.09 × 10−5
9.48 × 10−5
5.89 × 10−4
9.73 × 10−3
1.19 × 10−1
The value of jo increases for all three electrodes after applying CVs, which confirms again that the applied CVs improve the electron transfer rate (or decreases Rct). The jo values are in this order: Pd/SWCNTs@MOF-199-CPE > MOF-199-CPE > CPE. In right panel of the Fig. 11, the LSVs after applying CVs are magnified. According to these LSVs it is obvious that the lowest onset potential is for Pd/SWCNTs@MOF-199-CPE (about 0 V vs. RHE).
The results obtained from polarization and EIS studies both showed that the Pd/SWCNTs@MOF-199-CPE has the best performance related to CPE and MOF-199-CPE. The reason of this observation may be related to the presence of Pd and SWCNT in the composite structure. There are several reports which have used Pd as a catalyst for electrochemical hydrogen production [35, 36], but this metal is expensive, therefore, in this work the loaded Pd on the catalyst is low. Also there are some reports which have used carbon nanotubes in the electrode composition for electrochemical HER [37, 38, 39]. In addition, there are several reports which have applied the MOF [22, 23, 24, 25], Pd [40, 41] and SWCNT [37, 38] as adsorbent for hydrogen, and one of the famous mechanisms for electrochemical HER in acidic media is based on hydrogen adsorption on the electrode surface in the rate determining step (Volmer reaction) . Therefore, these materials by improving hydrogen adsorption can facilitate the HER progress.
In comparison with the first report for electrochemical HER with polyoxometalate-based metal organic frameworks (POMOFs) , although the onset potential is relatively higher, but it seems that the j value for the proposed electrode is greater. Also compared to two recently published reports for electrochemical HER on MOF-5 modified CPE  and Ni-based MOF modified GCE , the proposed modified CPE electrode has better current densities or onset potential for HER.
In this study, a nanocomposite of MOF-199 with SWCNT and Pd was synthesized and its characteristics was evaluated by XRD, BET, TGA and SEM. According to the rare reports for application of MOFs in electrochemical hydrogen generation, the performance of this composite as an electrocatalyst in CPE was studied, and some important parameters were optimized. The results show that this modifier has a good performance for HER, which compared to MOF-199 and bare CPE has the best performance. Also electrochemical studies showed that applying successive CVs or a constant potential on the electrodes surface does not decrease the electrode performance, but improves the electrocatalytic properties.
We gratefully acknowledge the partial support of this work from the Research Council of the Iran University of Science and Technology.
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