Alkaline ethanol oxidation on porous Fe/Pd–Fe nanostructured bimetallic electrodes

In this work, to prepare nanostructured and porous Fe/Pd–Fe bimetallic catalysts, the iron coating is applied firstly onto the copper substrate by the electrochemical deposition method. Subsequently, iron-zinc alloy coating is deposited on the underlayer iron. Eventually, by immersing this alloy coating in an alkaline solution containing palladium ions, the palladium will replace the zinc, resulting in porous Fe/Pd–Fe catalysts. The X-ray diffraction (XRD) technique was used for the characterization of the physical properties of the as-prepared electrocatalysts. Their electrocatalytic activity was studied by electrochemical methods such as cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). The XRD results showed that the zinc element was the main component of the Fe/Zn-Fe alloy and was replaced by palladium as a result of leaching-galvanic replacement. The electrochemical investigations showed that a new porous Fe/Pd-Fe bimetallic catalyst had higher electro-catalytic activity and stability than pure Pd and Fe electrodes for ethanol electro-oxidation in alkaline media. The superiority of the Fe/Pd–Fe catalyst is related to the high surface area and a synergistic effect between Fe and Pd in Fe/Pd–Fe catalysts. Therefore, the nanostructured Fe/Pd–Fe catalysts can be proposed as potential anode materials for alkaline ethanol fuel cells. A new porous Fe/Pd–Fe bimetallic catalyst shows higher electro-catalytic activity and stability than pure Pd and Fe electrodes for ethanol electro-oxidation in alkaline media. The superiority of the Fe/Pd–Fe catalyst is related to the high electrochemical surface area and a synergistic effect between Fe and Pd in Fe/Pd–Fe catalysts

The preparation of porous nanostructured materials by a simultaneous process of electrodeposition and galvanic replacement reaction has attracted considerable attention worldwide. This synthesis technique of the catalyst allows the change of crystallite sizes and thus controls their physical and chemical properties [6,9,10,[26][27][28][29].
We previously synthesized a porous nanostructured Fe/ Pd-Fe electrode [28] and systematically studied methanol electrooxidation. For the first time, we used it for ethanol electrooxidation in an alkaline solution. The catalytic activity of the synthesized electrodes toward ethanol electrooxidation was tested by cyclic voltammetry, chronoamperometry, and impedance techniques.

Fabrication of Fe/Pd-Fe electrodes
Porous Fe/Pd-Fe bimetallic catalysts were prepared according to a previous publication [28]. Briefly, Fe coating was first deposited on the copper substrate and was then coated by the iron-zinc alloy. Next, the Fe/Pd-Fe electrode was prepared by immersing the alloy coating of Fe-Zn in an alkaline solution of the corresponding Pd salt (1 mM of PdCl 2 + 30 wt.% NaOH) according to Eqs. (1) and (2): (1) Zn + PdCl 2 → Pd + Zn 2+ + 2Cl − After 48 h of immersion, the catalyst was finally removed from the above solution and washed three times with doubledistilled water.

Instruments and measurements
The crystalline structures and phases of the synthesized electrodes were characterized by X-ray diffraction (XRD) using an X-ray diffractometer (model APD 2000) using a Cu-Ka radiation source (wavelength = 0.15406 nm).
Electro-catalytic experiments were performed using a standard three-electrode cell arrangement with a vertex potentiostat/galvanostat (Ivium Technology, the Netherlands) electrochemical analyzer operated by the Ivium-Soft software. Pure Fe, Pd, and porous Fe/Pd-Fe bimetallic electrodes with a geometric surface area of 1 cm 2 were applied as working electrodes. An Ag/AgCl/KCl (saturated) electrode was utilized as a reference, and a large Pt sheet (about 20 cm 2 ) was utilized as a counter electrode. Cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) tests were done in a 1 M NaOH solution in the absence and presence of ethanol. The frequency range of the EIS measurements was 100 kHz-10 mHz. The current densities were normalized by electrochemical surface area (ECSA) of electrodes for uniformity. Also, all potentials were standardized to the values with reference to the reversible hydrogen electrode (RHE) using Eq. (3):

Characterization
The SEM images and the EDX spectra of the synthesized material were previously reported elsewhere [28]. In general, many cracks and pores are seen as a type of island morphology in the SEM image and these pores and cracks provide a high active surface area for the oxidation of ethanol [28]. The EDX analysis demonstrated that the amount of zinc was significantly reduced after selective leaching and the presence of Pd nanoparticles in the Fe/Pd-Fe sample, which confirms the exact synthesis of this catalyst [28]. Based on the EDX results and difference of electrode weight before and after leaching-galvanic replacement process, the mass of active Pd deposited is estimated to be 5 mg cm −2 for Fe/ Pd-Fe.
(2) Fe + PdCl 2 → Pd + Fe 2+ + 2Cl −  . Therefore, the main part of the Fe/Fe-Zn sample was zinc. The peaks of Fe were not found in the standard XRD due to strong Fe-Zn (001) texturing.
Five major lines of the X-ray diffraction scans of Fe-Pd nanoparticles can be seen around 2θ = 40.02°, 42.42°, 60.74°, 67.31°, and 77.46°. The absence of obvious peaks from impurity indicates the high purity of the obtained Fe/ Pd-Fe alloy. The appearance of Pd diffraction peaks indicates that Pd could be incorporated on the surface of the alloy instead of being doped into the lattice of the alloy successfully.

Cyclic voltammetry
The CVs of the smooth Fe, flat Pd, and porous Fe/Pd-Fe bimetallic electrode the smooth Fe, flat Pd, and porous Fe/Pd-Fe bimetallic electrode in the pure 1 M NaOH solution were studied in previous work [28]. For the smooth Fe electrode, the ECSA was estimated through a series of CV scans at different scan rates (20,40,60,80, 100, 120 mV s −1 ) under a non-Faradaic region (− 0.9 V to − 0.8 V vs. Ag/AgCl, (Fig. 2a)). The average difference between the anodic and cathodic charging current densities (ΔI/2 = (I a − I c )/2) in the middle of the potential window of CV curves was plotted as a function of scan rate (Fig. 2b). Then, the double-layer capacitance (C dl ) was found by obtaining the slope of the linear fit. The ECSA of Fe electrode was estimated by dividing the C dl found by the specific capacitance (C s ) according to the equation of (4): The specific capacitance was (0.040 mF cm −2 ), commonly used for metals in 1 M NaOH [30][31][32][33]. ECSA value of smooth Fe electrode is obtained to be 10.45 cm 2 .
The ECSA for flat Pd and porous Fe/Pd-Fe electrodes is calculated by dividing the coulombic charge (Q, mC) that determined by integrating of the reduction peak of PdO [34] in 1 M NaOH solution as shown in inset of Fig. 4b and 4c of previous work [28] by the required charge (0.405 mC cm −2 ) to reduce a monolayer of PdO using Eq. (5) [35,36]:  The CVs of these electrodes in a 1 M NaOH + 0.5 M C 2 H 5 OH solution are shown in Fig. 3.
The results show that three electrodes have electrocatalytic activity for ethanol oxidation in the alkaline solution. In the smooth Fe electrode, a cathodic peak at 0.42 V, an anodic peak at 0.88 V, and a broad shoulder belong to hydrogen evolution, hydrogen desorption, and ethanol electrooxidation, respectively [10,23,28].
Two oxidation peaks in the forward and backward sweeps are the typical behavior of ethanol oxidation on pure Pd and porous Fe-Pd bimetallic catalysts. For ethanol oxidation on the flat Pd and Fe-Pd bimetallic electrodes, the CV results show a similar voltammetric characteristic. The only difference is that the primary oxidation peak is very large, and the secondary oxidation peak is not seen for the Fe-Pd catalyst compared to the flat Pd electrode. Table 1 shows the onset potential (E s ), peak potentials of forward and reverse (E f , E b ), forward and reverse peak current density (I f , I b ), and the ratio of the (I f ) to (I b ), (I f /I b ) obtained from the CV analysis in Fig. 3.
The I f /I b is utilized to appraise the endurance of the electrode to pile carbonaceous species. The higher the I f /I b ratio is assigned to the more efficient elimination of more carbonaceous species on the electrode surface. The I f /I b ratio caused by ethanol oxidation on the porous Fe-Pd electrode is more significant than that of the flat Pd electrode. This very high I f /I b ratio for the Fe-Pd electrode as compared to flat Pd (~ 1.29) shows good endurance of the porous Fe-Pd bimetallic catalyst to intermediate species such as CO. In other words, it can be argued that ethanol is almost directly electro-oxidized to carbon dioxide on the Fe/Pd-Fe catalyst in the forward sweep. In the forward scan, the onset potential of ethanol oxidation on the porous Fe/Pd-Fe bimetallic nanostructured catalyst is 0.264 V, which negatively shifts by ∼0.11 V compared to that of the flat Pd electrode (0.374 V), and the smooth Fe electrode (∼0.825 V). This means that the overpotential in ethanol oxidation can be significantly decreased by the nanostructured Fe/Pd-Fe alloy [25,26]. The peak current densities are 2.008, 1.147, and 0.077 mA cm −2 for Fe/Pd-Fe, Pd, and pure Fe electrodes, respectively. Therefore, the nanocrystalline Fe/Pd-Fe alloy electrode has better catalytic activity than Pd and pure Fe electrodes for ethanol electrooxidation in alkaline solutions. The high electrochemically active area produced by the leaching-galvanic replacement process, smaller size, a synergistic effect between Pd and Fe, and higher dispersion of Pd nanoparticles in Fe/Pd-Fe are prominent factors regarding the higher catalytic activity of this catalyst than flat Pd and pure Fe. All results exhibit that porous Fe/Pd-Fe nanostructured alloys are the favorable electro-catalyst candidate for use in alkaline DEFCs. Table 2 compares the performance of Fe/Pd-Fe bimetallic catalysts with the alkaline ethanol oxidation catalysts reported in the literature [23,[37][38][39][40][41][42][43]. As shown in Table 2, onset potential of ethanol oxidation on nanostructured Fe/ Pd-Fe is lower than that on the literature catalysts and the peak current density is comparable to these catalysts. In other words, the synthesized Fe/Pd-Fe electrode has the best electro-catalytic properties toward ethanol electro-oxidation in an alkaline medium.  Table 1 The onset potential (E s ), peak potentials (E f , E b ), peak current density (I f , I b ) and I f /I b of the studied catalyst obtained from the CV analysis of Fig. 3 Electrocatalysts Onset potential (E s ) The effect of the scan rate of potential on the CV behavior of the porous Fe/Pd-Fe nanostructured bimetallic catalyst in the 1 M NaOH + 0.5 M C 2 H 5 OH solution is shown in Fig. 4a. It can be seen that, forward peak current density increases, and forward peak potential shifts to positive values with increasing scan rate. Figure 4b shows the plot of forward peak current density versus the square root of the scan rate for the porous Fe/Pd-Fe bimetallic catalyst. The linear relationship of I f with the ν 1/2 shows that the diffusion process only controls ethanol electro-oxidation. The positive shift of the peak potential with the ν 1/2 indicates that the ethanol oxidation reaction is irreversible on the surface of the bimetallic Fe/Pd-Fe catalyst. Figure 5 shows the effect of ethanol concentrations on the anodic current of ethanol oxidation for the bimetallic Fe/ Pd-Fe catalyst. As the concentration increases, the forward peak potential does not shift. Figure 5b shows that the anodic peak current density increased linearly with the concentration. The order of reaction for ethanol is obtained from the slope of this straight line according to Eq. (6) [9, 10, 44, 45]: In the above equation, I p , k, C, and n are the peak current density, the reaction rate constant, the bulk concentration of ethanol, and the reaction order, respectively. The values of the reaction order and reaction rate constant of the bimetallic Fe/Pd-Fe catalyst are 0.5149 and 0.0037, respectively (Fig. 5c).
The CV curves of the bimetallic Fe/Pd-Fe catalyst recorded at 50 mV s −1 in the temperature range of 25-70 °C in the 1 M NaOH + 0.05 M C 2 H 5 OH solution are shown in (6) Rate ≡ I p = kC n , logI p = logk + nlogC Fig. 6a. Accordingly, the anodic current density increases with temperature. A positive shift in the peak potential of the forward and reverse oxidation, and a negative shift in the onset potential are observed in ethanol oxidation based on the increase in the temperature on the porous nanostructured Fe/Pd-Fe catalyst. An Arrhenius plot of log I f vs. T −1 for the bimetallic Fe/Pd-Fe catalyst is shown in Fig. 6b. The electrochemical activation energy of ethanol oxidation was calculated with Eq. (7) [9, 28, 44]: In the above relationship, T, R, E a , and I f are temperature in Kelvin, the gas constant (8.314 J K −1 mol −1 ), activation energy in J mol −1 , and forward peak current density in A cm −2 , respectively. The 32.84 J mol −1 is the activation energy calculated from the slope of the curve.
A comparison of the electrochemical performance of ethanol oxidation with methanol oxidation (ref. [28]) on the flat Pd and porous Fe/Pd-Fe electrode is summarized in Table 3.
The onset potential (E s ) of methanol oxidation is 0.532 V on the flat Pd electrode, and it negatively shifts to 0.509 V on the Fe/Pd-Fe electrode. The onset potential for ethanol oxidation is 0.374 V on the flat Pd electrode and it negatively shifts to 0.264 V on the Fe/Pd-Fe electrode. About 23 and 110 mV reductions on the onset potential for the anodic reaction are significant for direct methanol and ethanol fuel cells, respectively. The results indicate that the enhancement in the activity for alcohol oxidation on Fe/Pd-Fe is due to the increase in the surface area and a synergistic effect by the interaction between Pd and Fe.  This work The peak current densities of methanol and ethanol oxidation on Fe/Pd-Fe are about two times higher than that on flat Pd. Moreover, the current at 0.72 V on Fe/Pd-Fe is about one and half that on flat Pd. The results show that Fe/ Pd-Fe has higher catalytic activity for methanol and ethanol electro-oxidation than flat Pd in alkaline media.

EIS test
The equivalent circuits and Nyquist diagrams of Fe/Pd-Fe, flat Pd and smooth Fe electrodes recorded at an anodic potential of − 0.20 V and in the 1 M NaOH solution are shown in Fig. 7. A serial model with two constant-phase elements (CPEs) shown in the inset of the Nyquist diagram (Fig. 7) was applied to fit impedance spectra obtained for the Fe/Pd-Fe electrode. In this model, R s , R P , and R ct denote the solution, pore, and charge transfer resistances, respectively. For the flat Pd and smooth Fe electrodes, the model of the equivalent-circuit includes the CPE in parallel with the R ct because only one semicircle is seen in its corresponding Nyquist diagrams.
The equivalent-circuit parameters and values of the roughness factor (R f ) of the synthesized electrodes obtained by fitting the EIS results in Fig. 7 are shown in Table 4.
The roughness factor can be calculated using the relationship R f = A real /A geometric . The real surface area for the electrodes is A real = (average double-layer capacitance (C dl ) of test electrode)/(C dl of a smooth metal surface) = 20 µF cm −2 [46][47][48][49]. The capacitance parameter T dl of the Fe/Pd-Fe electrode is related to the C dl by Eq. (8) [49]: In the case of the smooth Fe electrode, Eq. (4) changes slightly and C dl is obtained as follows [49]: In Eqs. (8) and (9), Φ is the phase shift that is considered a degree of surface homogeneity. The lowest values of R ct (33.68 Ω cm 2 ) and CPE-Φ (0.76) belong to the porous Fe/Pd-Fe electrode compared to the flat Pd and smooth Fe electrodes. This means that the Fe/Pd-Fe electrode shows higher electrocatalytic activity than smooth Fe and flat Pd. The mean error of fitting is also smaller than 3%, indicating the selection of a correct equivalent circuit and good fitting of the experimental data. EIS measurements are in agreement with the CV results.

Duration test
In an alkaline medium, the stability of the catalyst is also an essential parameter of the electrochemical process [28,[50][51][52]. The electrochemical stability and durability of the porous nanostructured Fe/Pd-Fe catalyst, pure Fe, and flat Pd were investigated by chronoamperometry at 0.77 V vs. RHE for 3600 s in a solution of 0.5 M C 2 H 5 OH in 1 M NaOH (Fig. 8).
All electrodes show a positive current density after 3600 s. The chronoamperometry curves show the initial sharp decay of current followed by a quasi-steady state. The current decline for the Pd and pure Fe electrodes is faster than the porous Fe/Pd-Fe bimetallic catalyst. The decreasing current density is mainly related to the partial poisoning of the catalyst with incomplete oxidation products adsorbed on its surface or more accumulation of carbonaceous intermediates. After 3600 s, the steady current   (Fig. 9). According to Fig. 9, the forward peak current density in the 500th cycle (2.33 mA cm −2 ) decreases very low compared with the 1st cycle (2.58 mA cm −2 ), which further confirms the superiority of the catalyst.
The antipoisoning and stability of the Fe/Pd-Fe electrode in the 1 M NaOH + 0.5 M C 2 H 5 OH solution were investigated Extreme by the EIS test of this electrode in the forward peak potential (0.95 V) after the cyclic voltammograms of the 1st and 500th cycle (Fig. 10). Table 5 shows the equivalent circuit parameters of the Fe/Pd-Fe electrode by fitting of EIS results from Fig. 10. It is clear that the charge transfer resistance after 500 cycles (8.93 Ω cm 2 ) has increased very little compared to the first cycle (7.63 Ω cm 2 ), which indicates the superiority and high stability of the electrode in an ethanol-containing alkaline solution.
Thus, these results indicate that the incorporation of Fe in the Pd matrix prevents the poisoning of active Pd sites more efficiently, and Pd sites play a prime role in improving the efficiency of the catalyst.

Conclusions
Nanocrystalline, porous, and Fe/Pd-Fe-based electrocatalysts were successfully synthesized using the electrodeposition and galvanic replacement route. The XRD results demonstrated the absence of obvious peaks from impurities. In general, the evidence of XRD, SEM, and EDS co-confirms that the Fe/Pd-Fe coatings are   The CV study showed that the current density of the bimetallic Fe/Pd-Fe catalyst was 2.008 mA cm −2 , which is about 2 and 26 times higher than those of flat Pd (1.147 mA cm −2 ) and pure Fe (0.077 mA cm −2 ) electrodes, respectively. The CA and EIS studies exhibited that the bimetallic Fe/Pd-Fe catalyst was superior to those of pure Fe and flat Pd for ethanol electrooxidation under the alkaline condition. This enhanced catalytic activity was created using Fe to Pd to oxidize adsorbed species similar to CO from surfaces. In general, all results show that the leaching-galvanic replacement process produces a highly porous catalytic surface suitable for use in ethanol electro-oxidation. Furthermore, the Fe/Pd-Fe catalysts are known to be very effective anode materials for ethanol oxidation in NaOH compared to pure Fe and flat Pd electrodes.