Fe–Pd nanoflakes decorated on leached graphite disks for both methanol and formic acid electrooxidation with excellent electrocatalytic performance

This paper introduces a unique and simple method for fabricating of inexpensive electrocatalysts for use in direct methanol fuel cells. The leached Fe1–Pd1 NFs/graphite (leached Fe1–Pd1/graphite) disk electrode was successfully obtained via uniform dispersion of Zn powder into the matrix of commercial graphite powder (98%), pressing under optimized pressure followed by the treatment in H2SO4 solution containing Fe+2 and Pd+2 cations, leading to the partial leaching out of Zn from graphite matrix, as well as partial electroless substitution of Fe–Pd nanoflakes with Zn metal. Based on the morphology studies, binary Fe–Pd nanoflakes with a large surface area uniformly dispersed on the leached graphite disk. The leached Fe–Pd/G disk showed the exceptional electrocatalytic activity toward methanol and formic acid oxidation without electrocatalyst poisoning being observed, in contrast to the leached Pd/graphite and leached Fe/graphite disks. This is due to the high surface area, and synergistic effect of Pd and Fe. The findings of this work may be used for the mass manufacture of graphite-based disks for commercial fuel cell applications using available graphite powders. The linear range of washed Fe1–Pd1/G electrocatalyst for measuring methanol was about 0.1–1.3 M, and its detection limit was calculated at about 0.03 M. Furthermore, the linear range of the nanocatalyst for measuring formic acid was about 0.02–0.1 M, and its detection limit was calculated at about 0.006 M.


Materials and methods
Powdered graphite (98%), Powdered Zn, Iron (II) chloride (FeCl 2 ), Iron(II, III) oxide (Fe 3 O 4 ),, Formic acid (HCOOH), sulfuric acid (H 2 SO 4 98%), Sodium hydroxide (NaOH), hydrochloric acid (HCl 37%), Oleic acid (C 17 H 33 COOH), Sodium Citrate(Na 3 C 6 H 5 O 7 ), ethanol (C 2 H 5 OH 99.7%), Palladium chloride (PdCl 2 ) and methanol (CH 3 OH 99%) were purchased from Merck (Darmstadt, Germany).The production of graphite disks included the use of the CARVER type 3925 hydraulic press machine, which was manufactured in the United States.In addition, electrochemical measurements were conducted using an Iranian SAMA 500 electroanalytical instrument manufactured in Isfahan.Throughout the whole of the investigation and for all solutions, a solvent consisting of double-distilled water, sourced from the DD Water Company in Iran, was used.In all experiments, a three-electrode system, including a leached graphite disk working electrode with an average cross-sectional area of 0.09 cm 2 , a platinum auxiliary electrode, and a reference electrode saturated with Ag/AgCl (Azar Electrode Company, Iran) was used.

Fabrication of the leached and non-leached graphite disks
To prepare graphite disks, 0.7 g of graphite powder was mixed with 0.3 g of Zn powder, and it was ground in a porcelain mortar for 15 min to make the mixture uniform.Using a press machine to apply 18,000 Pascal of pressure to the mixed powder, a graphite disk with a one-centimeter diameter and 0.3-mm height was produced.The graphite disk was then heated in an electric furnace for 12 h at 150 degrees Celsius.Two hours of soaking in a 1 M sulfuric acid solution increased the electrode's surface area by dissolving some Zn on the disk's surface, making it porous.The manufactured graphite disks were rubbed with sandpaper to conduct electrochemical experiments and measure the cross-sectional area of electrodes encased in plastic coverings.The cross-sectional area of the electrodes was determined to be 0.09 cm 2 .The non-leached graphite disk was made in the absence of Zn, as well as the leached graphite disk based on the above method.In the next step, nanoparticles containing Fe and Pd were easily deposited on the porous graphite disk surface via the electroless method.

Fabrication of the leached(porous) Fe-Pd/G electrocatalyst
For preparing leached Fe-Pd/graphite (porous Fe-Pd/G) electrocatalyst, a mixture of 0.02 M of Fe 3 O 4 and 0.005 M of PdCl 2 was prepared in HCl solution under sonication for 30 min to obtain the source of uniform Pd +2 and Fe +2 solution.Then, to deposit Fe and Pd nanoparticles on the surface of graphite disks, the prepared previous leached (porous) graphite disks were placed inside Pd +2 and Fe +2 solution for 1.5 h.The reactions performed in the electroless process are presented in below, where Pd +2 and Fe +2 can be reduced by extra Zn available in the surface of porous graphite disk.

Investigating the morphology of the synthesized electrocatalysts
Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) methods were used to examine the surface morphology of produced electrocatalysts, as shown in Fig. 3 SEM pictures acquired from electrocatalysts (a) non-leached graphite (b) the leached graphite (c) The non-leached graphite electrode was shown as Fe 1 -Pd 1 /G. Figure 3a There is no porosity, but the leached graphite disk in Fig. 3b clearly shows the porosity due to the placement of graphite disks in sulfuric acid and the Zn dissolving on the surface of the graphite disks, which increases the porosity.For the oxidation process, methanol was added to the solution containing the electrode.In Fig. 3c, the compact, and non-leached structure of the leached Fe 1 -Pd 1 /G disk can be seen, completely covered by the Fe-Pd layer.SEM findings show that a highly leached catalytic surface for use in methanol electrooxidation was produced due to the deposition, and substitution of iron with zinc in the base medium.TEM picture of the surface of the graphite electrode treated with Fe-Pd nanoflakes is shown in Fig. 3d.Based on the TEM data, the fabricated nanocatalyst has an average particle size of 40 nm, resulting in increased active surface area.The results of the the Brunauer-Emmett-Teller (BET) study showed a larger specific surface area.BET isotherm and Barrett, Joyner, and Halenda (BJH) (Barrett, Joyner, and Halenda) analyses of the pore size distribution of a leached and non-leached graphite disk are shown in Fig. 3e,f.The surface area, volume, and pore diameter of the leached graphite were greatly enhanced after its treatment in sulfuric acid, as shown in Fig. 3e,f and (Table 1) 48 .

Investigation of electrocatalyst hydrophilic properties
Porosity is created, and the electrode's surface area increases, boosting the hydrophilic characteristic.Parts (a) and (b) of Fig. 4 show the computed contact angles of distilled water with the leached and non-leached graphite disk electrode surfaces, respectively.Due to the difference of 11 degrees in the average contact angles between the two electrodes, 64 degrees for the non-leached graphite electrode and 53 degrees for the leached graphite electrode, it is discovered that the modified leached graphite electrode has hydrophilic properties and surface area.Decreasing the contact angle of electrode can significantly improve the electrocatalytic activity of the electrode due to increase the wettability for electrooxidation of methanol molecules dissolved in water 48 .

Energy dispersive X-ray (EDX) elemental analysis and mapping
Figure 5A shows the results of EDX elemental analysis performed by the leached Fe 1 -Pd 1 /G nanocatalyst.As can be seen in Fig. 5A, the above nanocatalyst is composed of the elements zinc, iron and palladium, carbon and oxygen.The percentage of elements used in the leached Fe 1 -Pd 1 /G electrocatalysts is 1.99% iron, 7.44% zinc, 0.63% Pd, 48.19% oxygen and 41.75% carbon.Based on EDX results, using Pd (0.63%) less than Fe (1.99%) is  Increases Pd resistance to poisoning by catalytic surfaces 49 .In fact, the combination of iron and palladium, known as the electrode catalytic activity, performs a catalytic function to enhance the electrocatalyst sensitivity for methanol detection 50 .In other words, at the surface of the graphite electrode modified with Fe-Pd alloy nanoparticles, the electron transfers rate rises and the anode current increases for the oxidation of methanol.Compared to the electrocatalysts discussed above, the leached Fe 1 -Pd 1 /G catalysts exhibit a higher current density, which typically indicates an increase in the catalyst's active surface area for methanol oxidation.This increases the catalytic activity of the leached Fe 1 -Pd 1 /G and the efficiency of direct methanolic alkaline fuel cells.Based on Fig. 6A, using the ratios of (1:1) palladium to iron for the oxidation of methanol shows a higher electrocatalytic activity than other electrocatalysts, so to make a catalyst for the oxidation of methanol, the ratio of (1:1) palladium to Iron was selected.The comparison of the sensitivity of the leached electrode made for the oxidation of methanol with the electrodes prepared by previous researchers is given in (Table 2), which shows the higher sensitivity of this electrode compared to other electrodes.Then, the oxidation peak of formic acid appeared at low potential, which indicates the higher efficiency of this electrode.Figure 6B shows the cyclic voltamograms of methanol oxidation by the electrocatalyst leached Fe 1 -Pd 1 /G in (a) 1 M NaOH and (b) (1 M NaOH + 1 M of methanol).In curve 6(a), there is no anode peak in the absence of methanol, according to Fig. 6B.In curve 6(b), methanol anode peaks (forward and reverse) due to methanol oxidation, at 0.18 potentials, respectively, are there, but they are not noticed, and only a Pd reduction peak at the (− 0.1) volts potential is visible.Moreover, 0.45V, with a broad-shoulder anode peak in the positive scan in the potential range from − 0.4 to 0.4V, is related to an increase of 1 M of methanol in the electrolyte.reverse peak scan mainly in terms of the removal of carbon species in the forward scan not completely oxidized is related to the oxidation of newly adsorbed chemical species 51 .

Behavior of the leached Fe 1 -Pd 1 /G electrocatalyst for oxidation of methanol
Comparing cyclic voltammograms of Fe 1 -Pd 1 /G, Pt/C electrocatalysts for the methanol oxidation is shown in Fig. 7A.As can be observed, Fe 1 -Pd 1 /G electrocatalyst compared to Pt/C electrocatalyst for methanol oxidation showed a higher current intensity, which indicates better performance of Fe 1 -Pd 1 /G electrocatalyst compared to Pt/C electrocatalyst for methanol oxidation.Furthermore, cyclic voltammograms of Fe 1 -Pd 1 /G, G/Zn electrocatalysts for methanol oxidation were studied.ascan be seen in Fig. 7B, for non-porous G/Zn electrocatalyst, which   By using the slope of the graph above and using Laviron's theory, the number of electrons involved in the oxidation of methanol can be calculated: The adsorption charge measured from the voltammogram of leached Fe 1 -Pd 1 /G electrocatalyst is used to estimate the active site density by the following equation 57 : In this equation, SD is mass-specific site density, N is Avogadro number (6.023 × 10 23 sites per mol), n is the number of electrons, F is Faraday constant (96,485 C.mol −1 ), and s is the geometric area of leached Fe 1 -Pd 1 /G electrocatalyst electrode (0.09 cm 2 ).The estimated active site density per unit cm 2 of catalyst at different scan rates for leached Fe 1 -Pd 1 /G electrocatalyst electrode is shown in Fig. 8c.
The temperature effect in the methanol oxidation process as an important factor was investigated 58,59 .The cyclic voltammetry approach was used to examine the effect of temperature on the methanol oxidation by the leached Fe-Pd/G nanocatalyst in (0.2 M methanol + 1 M NaOH) at temperatures of 25, 30, 35, 40, and 45 degrees Siliceus concurred.Figure 9A shows that when the temperature rises from 25 to 45 °C, the peak's height rises as well, indicating that the rate of methanol oxidation has risen.Moreover, it may be said that a reduction in load transfer resistance has resulted from a rise in temperature.Additionally, Fig. 9B provides an Arrhenius curve for the methanol oxidation using the leached electrode Fe-Pd/G.The activation energy value for methanol oxidation was found to be 2.46 kcal/mol using the Arrhenius diagram's slope.In addition of catalytic activity, the long-term stability of electrocatalysts is a significant criterion for evaluating their practical use 60,61 .The cyclic voltammogram of the leached Fe 1 -Pd 1 /G electrocatalyst was obtained after 200 cycles and compared to the initial cycle in order to assess the stability of the electrode and its resistance to Carbon monoxide (CO) poisoning.Compared to the first cycle, the forward peak current density in cycle 200 is decreased by about (0.3 mA) as seen in Fig. 9c, indicating excellent activity and durability of the leached Fe 1 -Pd 1 /G electrocatalyst for methanol oxidation in alkaline media.
The chronoamperometric (CHA) curves for all fabricated disks are shown in Fig. 10A in (1 M NaOH + 1.0 M methanol) at a constant potential of − 0.2 V for 300 s.As can be seen from the figure, all fabricated disks possess high peak current density at the beginning of the CHA test, attributing to the double layer charging process of the available huge active sites on the disk surface.As time passes in CHA test, current density was significantly reduced for all electrocatalyst disks which can be attributed to occupy and block the active sites on surface of disks due to toxic species specially CO derived from methanol electrooxidation.Finally, the current density was almost remained stable, assigning to constant diffusion of methanol species from bulk to surface disk.As can be seen from the Fig. 10A, the leached Fe 1 -Pd 1 /G electrocatalyst-disk exhibits greater stable current density (14.23 mA cm −2 ) compared to other fabricated disks for the electrooxidation of methanol, showing its higher durability against CO poising effect 58,62 .
Figure 10B shows plot of current density against the t 1/2 of CHA test for leached Fe 1 -Pd 1 /G electrocatalystdisk, where diffusion coefficient can be calculated.The diffusion coefficient of methanol in aqueous medium can be calculated using the following equation, which is known as Cattrell's equation.In Cottrell's relation, I: current intensity in amperes, n: number of exchanged electrons, F: Faraday number, C: concentration of electroactive compound in t: time in seconds, D: diffusion coefficient in A: electrode surface area in is.Using the Cottrell equation and the slope of the flow diagram in terms of time, the diffusion coefficient of methanol (D) was obtained 7 × 10 −4 cm 2 .s−1 .
In electrochemical systems, the kinetics of electrode reactions can be studied using electrochemical impedance spectroscopy (EIS) 63 .In 1.0 M NaOH and 1.0 M CH 3 OH at 0.20 V, typical Nyquist plane plots of the     www.nature.com/scientificreports/resistance on the electrocatalysts, respectively 30 .Also, according to relation of i 0 = RT/FR ct (T is temperature, R is the gas constant and F is Faraday constant), exchange current density (i 0 ) is calculated.The below table shows a comparison of obtained electrodes in view of exchange current density (i 0 ) and charge transfer resistance (R ct ) 64 .As can be seen from the Table 3, leached Fe 1 -Pd 1 /G possess lower R ct and higher i 0 compared to other electrodes, showing its good electrocatalytic activity compared to other electrodes which is in accordance with other electrochemical evaluations.

Behavior of different fabricated electrodes for oxidation of formic acid
Figure 12A compares the cyclic voltammograms of various fabricated disk-electrocatalysts in 0.1 M H 2 SO 4 and 0.1 M formic acid at a scan rate of 20 mV/s.As observed in Fig. 12A, the leached Fe 1 -Pd 1 /G disk-electrocatalyst has maximum electrocatalytic activity towards formic acid oxidation originated from nanostructure morphology and the synergistic impact of Fe-Pd alloy.In actuality, the combination of iron and palladium serves as a catalyst to boost the modified electrode's sensitivity in oxidation of formic acid.Table 4 compares the sensitivity of the fabricated leached disk-electrocatalysts towards formic acid oxidation with other electrodes reported in previous works.As can be seen from the Table, the leached Fe 1 -Pd 1 /G disk shows more negative oxidation potential and higher current density towards formic acid oxidation compared to other previous electrodes.Figure 12B displays electrooxidation of formic acid on the leached Fe 1 -Pd 1 /G disk at various formic acid concentrations in 0.1 M H 2 SO 4 and 20 mV/s scan rate.As formic acid concentration increases up to 0.1 M, the peak changes slightly to higher positive values, and the anode peak current density increases continuously, suggesting a large number of active nanocatalyst sites.The curve of current versus concentration at 0.1 M formic acid concentration is shown in Fig. 12C, displaying linear correlation between concentration and peak current density.

Conclusion
The porous Fe-Pd NFs/G coin was prepared using facile electroless deposition of Fe-Pd nanoflakes onto porous Zn-graphite coin obtained from pressed commercial graphite and Zn powders.The porous Fe-Pd NFs/G showed enhanced electrocatalytic activity and good stability toward methanol oxidation with high anti-CO poisoning capability compared with leached Pd/G and Fe/G disks.The anodic peak for oxidation of methanol on porous   Fe-Pd NFs/G electrode was almost two and three times higher than that of other electrocatalysts such as porous Pd/G electrode and porous Fe/G electrode, respectively.Compared to similar electrocatalysts, the materials used in the current disk are available, cheap and non-toxic.Therefore, the use of these disks will be very helpful in the direction of developing the use of non-fossil and clean energy systems.The results of this research can be used in industries related to new energy, in fuel cell systems, batteries and supercapacitors.Also, these nanocatalysts can be used in electrocatalytic processes to detect and measure drugs and biological species.

Figure 1 .
Figure 1.X-ray diffraction spectrum of the leached Fe 1 -Pd 1 /G electrocatalyst Loaded on a graphite substrate.

Figure 4 .
Figure 4. Drop test images related to electrocatalysts (a) non-leached graphite and (b) The leached graphite.

Figure 11 .
Figure 11.(A) Nyquist plots of methanol electrooxidation on (a) bare graphite, (b) The leached Fe/G, (c) The leached Pd/G and (d) The leached Fe-Pd/G electrocatalysts in 1 M NaOH + 1 M methanol at an electrode potential of − 0.20 V. (B) Electrical equivalent circuits.

Figure 12 .
Figure 12. (A) Cyclic voltammograms of (a) The leached Fe 1 -Pd 1 /G, (b) bare graphite.(c) The leached Fe/G (d) The leached Pd/G electrocatalysts in (0.1 M H 2 SO 4 + 0.1 M formic acid) with a potential scan rate of 20 mV/s.(B) Cyclic voltammograms of formic acid oxidation by the leached Fe 1 -Pd 1 /G electrocatalyst in different concentrations formic acid of 0, 0.02, 0.04, 0.06, 0.08, 0.1M respectively from (a to f) in 0.1 M H 2 SO 4 with potential scan rate of 20 mV/s.(C) Plot of flow versus concentration at 0.1 M formic acid concentration.

Table 1 .
BET isotherm and BJH pore size distribution analysis of the leached and non-leached graphite disk.

Table 2 .
Comparative performance of this as-prepared sensor and some others for the determination of methanol.

Table 3 .
A comparison of obtained electrodes in view of exchange current density (i 0 ) and charge transfer resistance (R ct ).

Table 4 .
Comparative performance of this as-prepared sensor and some others for the determination of formic acid.