Fariba Kaedia,
Zahra Yavari*ab,
Ahmad Reza Abbasianc,
Milad Asmaeic,
Kagan Kermand and
Meissam Noroozifar*d
aDepartment of Chemistry, University of Sistan and Baluchestan, Zahedan, P.O. Box 98135-674, Iran. Fax: +98-54-3341-6888; Tel: +98-54-3341-6464E-mail: z_yavari@chem.usb.ac.ir; zahrayavari5@gmail.com
bRenewable Energies Research Institute, University of Sistan and Baluchestan, Zahedan, Iran
cDepartment of Materials Engineering, Faculty of Engineering, University of Sistan and Baluchestan, Zahedan, Iran
dDepartment of Physical and Environmental Sciences, University of Toronto, Scarborough 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada
First published on 23rd March 2021
Structure and surface area are critical factors for catalysts in fuel cells. Hence, a spinel nickel ferrite mesoporous (SNFM) is prepared via the solution combustion technique, an efficient and one-step synthesis. Dynamic X-ray analysis has clarified the structural properties of SNFM. The grain size of SNFM is determined to be ∼11.6 nm. The specific surface area (87.69 m2. g−1) of SNFM is obtained via the Brunauer–Emmett–Teller method. The Barrett–Joyner–Halenda pore size distributions revealed that the size of the mesopores in as-synthesized SNFM mainly falls in the size range of 2–16 nm. Scanning electron microscopy studies showed the regularities involved during porous-structure formation. SNFM is employed as the support for nano-structured palladium (PdNS). Field emission scanning electron microscope studies of PdNS-SNFM showed the deposition of PdNS in cavities and on/in the pores of SNFM. The electrochemical surface area obtained for PdNS-SNFM is about 27 times larger than that of PdNS via cyclic voltammetry. The electrochemical studies are utilized to study the features and catalytic performance of PdNS-SNFM in the electro-oxidation of diverse small organic fuels, whereas the electrooxidation of diethylene glycol is reported for first-time.
In the last decade, Pd has been considered in the oxidation processes19 because it exhibits an impressive electrocatalytic ability, and in contrast to Pt electrocatalysts, Pd catalysts has superb resistance to poisoning from the oxidation of alcohol.20 Moreover, the abundance of Pd is about 50 times more than Pt. For this reason, Pd is cheaper than Pt. Consequently, the fabrication of noble metal and mixed oxide on the nanoscale can be the essential factor affecting the efficiency of noble-metal electrocatalysts.21 Also, supported nano-particles have been explored in oxidizing conditions, and it has been demonstrated that an excessively thin surface oxide forms before the onset of bulk oxidation. This thin surface oxide is more active in oxidation than corresponding metallic surfaces.22
Here, the spinel nickel ferrite mesoporous (SNFM) was synthesized via a salt-assisted solution combustion method and characterized via a scanning electron microscopy, X-ray diffraction, and Brunauer–Emmett–Teller analysis. The nano-structured palladium (PdNS) in the presence of deacetylchitin as an adhesive agent on the electrode's surface23 was placed onto the pores of SNFM. Electrochemical studies were performed to investigate the features and catalytic performance of PdNS-SNFM during the oxidation of diverse small organic fuels (SOFs), including methanol (SOF1), ethanol (SOF2), ethylene glycol (SOF3), diethylene glycol (SOF4), formaldehyde (SOF5), and formic acid (SOF6); whereas diethylene glycol electrooxidation is reported for the first time.
(1) |
The electrochemical tests were performed by an SAMA 500 electroanalyzer (SAMA Center, Iran) in a cell containing: a Pt (as the auxiliary electrode) electrode, Hg/HgO (as the reference electrodes) electrode, and glassy carbon (GC, as the working electrode) electrode with 3.14 × 10−6 m2 surface area. The working electrode was prepared as follows: (1) mechanical polishing, (2) electrochemical activation in an acidic medium by an anodic and cathodic sweep, (3) spraying 10 μl of catalytic composite on the surface electrode, and (4) solvent evaporation of the composite and the creation of a catalytic layer on the surface electrode, which were denoted as GC/PdNS-SNFM and GC/PdNS electrodes.
Fig. 1 (A). XRD pattern, (B) FTIR spectrum, (C) nitrogen adsorption and desorption isotherms (inset t-plot), (D) BJH pore size distributions. |
Fig. 1B depicts the FTIR spectrum of SNFM powders. The broad band at around 3380 cm−1 is responsible for the presence of the O–H group.25 The weak bands at 2926 and 2853 cm−1 can be assigned to C–H asymmetric and C–H symmetric stretching frequencies, respectively.26 The band around 1633 cm−1 correspond to the vibration of the N–H bond.27 The band at 1393 cm−1 corresponds to NO3− ions.28 The observed weak band at 1090 cm−1 is due to CO stretching with ring stretching.29 The bands at 562 and 435 cm−1 indicate the cubic spinel structure formation of NiFe2O4. The band at 435 cm−1 proves metal–oxygen stretching in the octahedral position. The intense peak at 562 cm−1 is correlated to the metal-oxygen stretching tetrahedral band.30
The N2 adsorption/desorption isotherms of the obtained SNFM powder is presented in Fig. 1C. The hysteresis profile is detected to form IV class with H3 hysteresis, which suggests the presence of mesopores.31 Also, the t-plot of SNFM powders is shown in the inset of Fig. 1C. The t-plot depicts a sharp straight line that begins from the origin point but becomes smoother after some point. Therefore, SNFM powders belong to mesoporous materials. Likewise, SNFM powder exhibits a specific surface area and total pore volume of 87.69 m2 g−1 and 0.2377 cm3 g−1, respectively. The BJH pore dimension distributions are also depicted in Fig. 1D, which shows that the size of mesopores in the as-synthesized SNFM mainly falls in the range of 2–16 nm. The presence of slit-shaped pores and panel-shaped particles generates the H3 hysteresis. Due to non-rigid particle aggregates, the isotherms revealing type H3 do not limit adsorption at high P/P0.31 Consequently, it seems that the brittle agglomerations of porous particles result in IV isotherms with H3 hysteresis.
In the SEM micrograph with a lower magnification (Fig. 2A), a porous and spongy structure of SNFM is observed. In another image with higher magnification (Fig. 2B), the effect of combustion is more apparent. The combustion created tiny cavities in SNFM powders. Also, it can be seen that agglomerates are formed by assembling nanoparticles. Therefore, Pd can be inserted on the surface and in the cavities of SNFM.
Fig. 2 The SEM images with (A) 10, and (B) 1 μm scales for SNFM; (C) FESEM image of PdNS-SNFM, with 100 μm scale, and (D) TEM micrograph of PdNS-SNFM with 50 μm scale. |
To consider the morphology of PdNS-SNFM, FESEM images are shown in Fig. 2C. It is clear that the holes were occupied with PdNS (brighter points) in the SNFM structure (darker places). Therefore, the formation of PdNS into holes and onto the surface of SNFM was demonstrated. Nevertheless, there are unfilled cavities in PdNS-SNFM.
Such cavities improve the fuel storage on the surface catalyst for electrooxidation. The good dispersion of PdNS on SNFM is marked.
Fig. 2D displays the TEM micrograph for PdNS-SNFM in 50 nm scale. It confirmed that the palladium particles formed in nano-size with diameter range of 10–20 nm. As in the FESEM image, the unfilled holes in the TEM image are visible.
The electrochemical surface area (EASH/EASPd) is important to define electrocatalyst efficiency.35 EASH and EASPd are respectively estimated over the surface below the peak adsorption/desorption hydrogen (QH) and the surface below the curve of Pd–O reduction (QPd).36,37 The durability of the as-synthesized electrocatalysts was examined by applying potential cycling in an alkaline medium. Comparisons of EASH and EASPd at the beginning and end of the test were used to analyze the loss EAS rate. QH (1th cycle = 104.18 and 80th cycle = 103.28 mC cm−2) and QPd (1th cycle = 127.39 and 80th cycle = 122.36 mC cm−2) were obtained on GCE/PdNS-SNFM, while QH (1th cycle = 4.00 and 80th cycle = 2.66 mC cm−2) and QPd (1th cycle = 7.04 and 80th cycle = 4.01 mC cm−2) were on GCE/PdNS. This data exhibited that EASH and EASPd for PdNS-SNFM were considerably improved relative to PdNS. The dispersion of PdNS was dropped after durability for modified electrodes, representing decrease in EASH and EASPd due to sintering and dissolution of PdNS. The loss percentages were 33.55 and 9.75 for PdNS and 1.26 and 3.94 for PdNS-SNFM on EASH and EASPd, respectively. These data are presented in Table S1.† A comparison showed that using a mesoporous support enhanced the dispersion and electrochemical surface area of PdNS. PdNS-SNFM had superior distribution and durability compared to PdNS. SNFM with three-dimensional porous construction can be sufficient for inhibiting sintering, agglomeration and, dissolution of PdNS. Therefore, the stability of the electrocatalyst can be improved through SNFM. The size and dispersion of PdNS were better due to the creation of PdNS on and into the pores of SNFM as the support.
Also, the stability of PdNS-SNFM and PdNS were studied via chronoamperometry (CA). Fig. 3B shows the chronoamperograms of SOF1–SOF6 oxidation on the PdNS-SNFM catalysts at 0.1 V potential for 300 s. In the beginning, I vs. t curves meaningfully dropped (about 10 first seconds). Next, the current density was approximately stable owing to the falling number of active sites on the electrocatalyst to substitute the SOF molecules. Such enhanced stability is explained via the improved creation of OH groups on the SNFM surface.38 The reaction among such groups and middle species may increase the durability of PdNS-SNFM. As shown in Fig. 3B, SOF5 has a greater current density (637.01 mA cm2) compared to the other SOF.
The number of fuel molecules per electrocatalyst surface per second is equal to the turnover number (TON). This factor determines the current in steady-state for a given electrooxidation process. The number of fuel monolayers that are electrooxidized per unit time is known as the turnover frequency (TOF). These two factors can be estimated according to ref. 39. The electrochemical results from Fig. 3B were presented in Table S2.†
The electrooxidation reaction of SOFs on PdNS-SNFM was evaluated utilizing the CV technique. Fig. 3C shows the CV curves related to SOF1–SOF6 electrooxidation on GCE/PdNS-SNFM with 0.05 V s−1. The peak that appeared in the forward sweep (If) is ascribed to the oxidation of the adsorbed species including SOF. In contrast, the second peak in the backward sweep (Ib) is responsible for the oxidation of created intermediate components that were not completely oxidized in the forward scan.39 A likely pathway for the SOF oxidation reaction is expressed in Table 1.40–44
SOF1 (ref. 40) | SOF2 (ref. 41) | SOF3 (ref. 42) | SOF5 (ref. 43) | SOF6 (ref. 44) |
---|---|---|---|---|
Pd + CH3OH → Pd−CH3OHads | Pd + CH3CH2OH → Pd−CH3CH2OHads | Pd + (CH2OH)2 → Pd-(CH2OH)2 ads | Pd + H2O → Pd–OHads + H+ + e− | The general reactions |
HCOOH + Pd → HCOO−Pd + H+ + e− | ||||
HCOOH + Pd → Pd−CO + H2O | ||||
Pd−CH3OHads + OH− → Pd−CH3Oads + H2O + e− | Pd−CH3CH2OHads + 3OH− → 3H2O + Pd−CH3 ads + 3e− | Pd–(CH2OH)2 ads+ 4OH− → Pd–(HCO)2 ads+ 4H2O + 4e− | Pd + HCHOsol → Pd−HCHOads | Formic acid electrooxidation reaction in the direct pathway |
HCOO−Pd → Pd−H + CO2 | ||||
Pd−H → Pd + H+ + e− | ||||
Pd−CH3Oads + OH− → H2O + Pd−CH2Oads + e− | Pd−CH3COads + Pd−OHads → Pd + Pd−CH3COOH | Pd−HCO2 ads + 4OH− → Pd−HCOO2 ads + 2H2O + 4e− | Pd−HCHOads → Pd–CHOads + H+ + e− | Formic acid electrooxidation reaction in the indirect pathway |
Pd + H2O → Pd−OH + H+ + e− | ||||
Pd−CO + Pd−OH → Pd + CO2 + H+ + e− | ||||
Pd−CH2Oads + OH− → Pd−CHOads + H2O + e− | Pd−CH3COOH + OH− → Pd + CH3COO− + H2O | Pd–HCOOads + e− → Pd–COads + OH− | Pd–CHOads → Pd–COads + Pd–OHads + H+ + e− | Net reaction |
Pd−CHOads + OH− → Pd−COads + 4H2O + e− | Pd–HCOads + e− → Pd–COads + OH− | Pd–COads + Pd–OHads → Pd + Pd–COOHads | HCOOH → CO2 + 2H+ + 2e− | |
Pd−COads +2OH− → Pd + CO2 + H2O + 2e− | Pd−OH− → Pd + OHads + e− | Pd + Pd–COOHads → 2Pd + CO2 + H+ + e− | ||
Pd–COads + Pd + OHads → 2Pd + CO2 + H+ + e− |
The stability of the If/Ib ratio during 50 cycles is shown in the bar chart in Fig. 3D. It is responsible for the earlier elimination of intermediates on the PdNS surface due to the higher surface area. It indicates that the synergistic properties of SNFM on PdNS performance enhance the electrocatalytic ability of PdNS-SNFM as a multi-functional electrocatalyst in SOF electrooxidation. Structural oxygens in SNFM are the active species to eliminate the intermediates of SOF electrooxidation. Support containing interface metal like NiFe2O4 is a promoter agent for the dehydrogenation as the initial step of oxidation of small organic fuels due to their multi-oxidative state via the orienting C–H bond; because it can be desirable to create the cycling redox among the high and low valences.
The Ni3+/Ni2+ and Fe3+/Fe2+ redox couples exhibit good electrocatalytic performance. As stated before, the thin surface oxide formed is more active in oxidation than corresponding metallic surfaces.22 Presumably, the surface oxygen of ferrite activated the oxidation process. On the other hand, the porous network of NiFe2O4 prevents PdNS aggregation. The synergistic influence can be because of the earlier removal of intermediates due to increased surface area and weakened agglomeration of PdNS, thus decreasing the resistance of mass transfer to store small organic fuels into the holes of NiFe2O4 ceramic support.
The accessible sites on PdNS were particular for H2 adsorption during enhancing SOF concentration. Consequently, the exchange current was decreased. The degree of surface coverage (θ) determines the outcome of species competition in the adsorption. Hence, the range of potential for linear polarization the gradient of Langmuir isotherm displays the formation of the SOF mono-layer on the electrode surface. The SOF was adsorbed on the active sites of PdNS-SNFM without any interaction among them.46 The slope amounts of Langmuir isotherm for SOF1–SOF6 were not equal to the unit. Hence, it is clear that there are the significant interactions among absorbed species.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra10944d |
This journal is © The Royal Society of Chemistry 2021 |