High-performance IN738 superalloy derived from turbine blade waste for efficient ethanol, ethylene glycol, and urea electrooxidation

In this work, IN738 superalloy used previously in gas turbines was recycled and used as a working electrode for the electrooxidation of different fuels, namely ethylene glycol, ethanol, and urea. The electrocatalytic efficiency of the electrode was studied by cyclic voltammetry, chronoamperometry, and electrochemical impedance. Several kinetics parameters like diffusion coefficient, Tafel slope, rate constant, and activation energy were calculated. The modified electrode was characterized as received using XRD, SEM, and EDAX to elucidate the crystal structure and surface morphology before and after electrochemical oxidation. The anodic current densities of electrochemical oxidation of ethanol, ethylene glycol, and urea were 29, 17, and 12 mA.cm−2, respectively, in an alkaline solution at a potential of 0.6 V (vs. Ag/AgCl). The kinetic parameters like diffusion coefficients for ethanol, ethylene glycol, and urea were found to be 1.5 ×\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\times$$\end{document} 10–6, 1.038 ×\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\times$$\end{document} 10–6, and 0.64 ×\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\times$$\end{document} 10–6 cm2 s−1, respectively. The charge transfer resistances were estimated for electrooxidation of different fuels by electrochemical impedance spectroscopy (EIS).


Introduction
Fuel cells are a promising device that is widely used for energy generation that is clean and sustainable [1][2][3][4]. Moreover, the fuel cell is recognized as a device that generates green and sustainable electrical energy via the electrooxidation of a hydrogen-rich molecule (i.e., methanol, ethanol, formic acid, ammonia, glycerol, and urea) [5][6][7][8][9][10]. However, fuel cells have several profits over conventional combustionbased technologies used in many power plants and vehicles [11].
Fuels can be mainly classified depending on their physical state (solid, liquid, or gas), origin, and chemical structure. Several materials can be used as fuels, like ethylene glycol, ethanol, and urea [12][13][14][15]. Ethylene glycol (EG), a cheap and non-volatile material, has been interested in its lower toxicity and higher ignition point than other alcohol. Besides, the energy capacity of EG ca. 4.8 A.h.mL −1 is higher than methanol (4 A.h.mL −1 ) [16]. Based on the structure of EG, the complete conversion of EG to CO 2 evaluates ten e − per one molecule of alcohol, while the reaction can be expressed as follows (Eq. (1)) [17]: Ethanol is a non-toxic liquid produced from gasoline and biomass. Ethanol is alcohol with a C-C bond that has a higher energy density compared with methanol, since it can deliver 12 electrons per molecule if fully oxidized to CO 2, as illustrated in the following Eq. (2) [18]: Urea is an impressive fuel because of its ability to be produced in large scans by the petrochemical process. In addition, urea-rich water is available in the sludge of industrial processes and human urine in the wastewater. Direct urea fuel cells (DUFCs) are one of the most rapidly growing fuel cells. Nowadays, many researchers focus their efforts on DUFC owing to a high energy density of up to 17 MJL −1 compared with liquid hydrogen equal to 10 MJL −1 [19].
The urea electrochemical oxidation is considered as six electron process which can be represented by the following Eq. (3) [20]: Several precious metals like palladium and platinum are widely used as anodic materials for electrochemical oxidations due to their high stability, low CO tolerance, and high catalytic conversion [21][22][23][24][25]. Recently, the nickel-based catalyst has been considered a significant element for electrochemical oxidation, compared with other precious metals, like palladium and platinum. The growing interest in Ni-based catalysts is owing to their availability, low toxicity, and cheap cost.
Turbine blades are considered the most sophisticated single-crystal superalloys globally due to the continuous material with zero grains. At the same time, the superalloy can afford extraordinary mechanical properties under elevated temperatures and high-stress conditions [35]. Ni-based superalloys are widely used for high-temperature applications, such as disks and blades of either aerospace engines or land-based gas turbines [36][37][38][39]. The gas turbine blades used for power generation are mostly nickel-based superalloys, such as IN738LC alloy. Ni-based superalloy includes several elements in crystal systems, such as tungsten, tantalum, niobium, and molybdenum [40].
The high percentage of nickel element facilitates the nickel-based superalloy suitable for electrocatalysis in an alkaline medium. On the other hand, the presence of another transition element like Cr, Mo, and Nb was proved to reinforce the catalysis potential by enhancing the electrical properties and the poisoning resistance [41][42][43][44][45]. Therefore, the activity of IN738 alloy is expected to be high toward the oxidation of small molecules regarding the structure and chemical composition.
In this work, IN738 electrode derived from a waste gas turbine blade was used as anode material for the electrochemical oxidation of different organic molecules, like ethylene glycol, ethanol, and urea. A comparative electrochemical study was performed, and different kinetics parameters were calculated for the different fuel oxidation upon the working electrode.

Materials, chemicals, and solvents
IN738 waste alloy was received as an old turbine blade scrap from a gas turbine power plant. The chemicals used in the electrochemical experiment were analytical grade. All solutions were prepared with double distilled water.

Preparation of IN738 superalloy
The sample was first soaked in 0.5-M nitric acid to clean the surface of any organic or carbon materials. Then, the surface was polished with conventional sand polish paper. Finally, the electrochemical treatment was performed on IN738 sample by soaking the electrode in 1.0 M of KOH to generate the corresponding hydroxide species by cyclic voltammetry. Table 1 shows the chemical composition of the as received.

Structural and surface analysis equipment
The XRD was performed as a prepared IN738 superalloy using the Panlytical X'Pert instrument with Cu-Kα radiation (λ = 1.540Ǻ). The Quanta 250 FEG instrument was used for scanning electron microscopy (SEM) and EDAX measurements of IN738 sample. To enhance the samples' conductivity and resolution measurement, the samples were coated with gold using EMITECH k550x sputter coater.

Electrode preparation
The electrochemical oxidation of fuel was performed on the surface of a well-polished (mirror-like) IN738 electrode (cuboid shape alloy with dimension length, width, and height ca. 5, 5, 1 mm) sealed in an epoxy resin jacket, leaving an exposed geometric surface area of 0.25 cm 2 . For impurities removal, the IN738 electrode was immersed in 10% HNO 3 , rinsed with distilled water, and then polished using 2500 grit emery paper to obtain a smooth mirror-finish surface. The electrochemical studies were performed using cyclic voltammograms, chronoamperometry, and the electrochemical impedance to investigate the activity of IN738 electrode toward the electrooxidation of different fuels, like ethanol, ethylene glycol (EG), and urea. The three-electrode system was used to perform the required studies in a solution containing a 1.0-M potassium hydroxide aqueous electrolyte, a platinum wire as the counter electrode, and an Ag/ AgCl electrode (saturated KCl) as a reference electrode. Autolab workstation (PGSTAT128N) was employed to find the different electrochemical experiments, like cyclic voltammetry, chronoamperometry, and electrochemical impedance. Nova software (ver. 2.1) was used as a graphical user interface. The EIS experiment was measured by applying an AC Potential of 0.1 Hz up to 1 × 10 4 Hz. Because of the nature of the sample (metal waste), all electrochemistry results were measured three times to ensure the reproducibility of the data.

Surface characterization
The morphology and elemental analysis of IN738 surface were performed by scanning electron microscope (SEM) and energy-dispersive X-rays (EDX). Figure 2a represents the SEM image of the surface of IN738 before oxidation. Due to acid polishing and surface treatment, the tunnel appeared on the alloy surface. The surface was studied after the oxidation of urea. As shown in Fig. 2b, the metal hydroxide layer  covers the surface. On the other hand, surface deterioration was observed at the edges of the tunnels. Figure 3 represents the EDAX of IN738 surface. The elemental analysis of IN738 surfaces was estimated by EDAX analysis, as reported in Table 1.
According to a previous study of IN738 superalloy microstructure [48,49], the composite of the alloy consists of a Ni-based austenitic γ matrix, γ' precipitates, and MC carbide in addition to γ/γ' eutectic.
The grain size was expected to be 100 μm. According to the microstructure analysis, two types of carbides were represented in the superalloy located inside the grain boundaries of the γ matrix. The presence of (Ta, Ti) C in addition to a minor percentage of (Nb, W) C was proved by EDAX results.

Fuel electrooxidation
The electrochemical activity was studied for IN738 electrode toward oxidation of the different small molecules, like ethylene glycol, ethanol, and urea. IN738 is considered a supper alloy with high Ni-contained 50%. Otherwise, the presence of various transition metals in the alloy crystal enhances the catalytic efficacy of the alloy. The activity of the electrode was investigated by cyclic voltammetry (CV) in an alkaline medium of 1.0-M KOH at a scan rate of 20 mV s −1 .  The electrode was first activated in KOH to regenerate active species and form metal hydroxides. As shown in Fig. 4a, the electrode was exposed to 50 repeated cycles in a solution of 1.0-M KOH at a scan rate of 100 mV s −1 . Figure 4b shows the CVs of electrochemical oxidation of ethanol, ethylene glycol, and urea upon IN738 surface in a solution of 1.0-M KOH and 1.0 M of fuels at a scan rate of 20 mV s −1 . IN738 was the effective surface of the oxidation of these fuels. Consequently, the recognizable oxidation peaks were observed corresponding to the oxidation of each fuel. The higher electrode activity toward ethanol over ethylene glycol is attributed to its simple structure and highest diffusion rate. Additionally, the presence of two hydroxyl groups in ethylene glycol increases the interaction with the solvent and suppresses the molecule's diffusion toward the electrode surface. The adsorption of urea on the metal surface is due to the carbonyl group of urea molecules [51]. Despite the small structure of the urea molecule, it was observed to have the lowest current density owing to the small output electron of complete oxidation compared with ethylene glycol and ethanol counterparts. Furthermore, the direct oxidation of urea leads to the more defined redox peak, as represented in Fig. 4b. The electrochemical oxidation of ethylene glycol is expected to be more complex compared to the urea and ethanol counterparts. The difficulty of complete oxidation of ethylene glycol is observed by the undefined oxidation peak and the decrease of the Ni 2+ /Ni 3+ peak due to the accumulation of uncomplete oxidative carbon species on the nickel surface.
The activity of different nickel-based surfaces was compared with the result of IN738 for various fuels in an alkaline medium (see Table 2).
Tafel slope is one of the electrochemical parameters obtained from linear sweep voltammetry. The value of Tafel slope reflects how the electrochemical process can take place effectively. Thus, the lower value of Tafel slope indicates the smaller value of potential required to initiate the reaction. As shown in Fig. 4c, Tafel plot of IN738 electrode for ethylene glycol, ethanol, and urea was calculated as 105, 80.2, and 51.2 mV dec −1 , respectively. The charge transfer coefficients were also established from Butler-Volmer Eq. (5) [60]: where ɳ is the over-potential, R is the universal gas constant, T is the operating temperature, n is the number of electrons involved in oxidation (n ethylene glycol = 10, n ethanol = 12, n urea = 6), F: is the Faraday constant, and i is the oxidation current.
The charge transfer coefficient (α) was calculated as 0.92, 0.902, and 0.904 for ethylene glycol, ethanol, and urea, respectively. The comparison between electrode activities for different fuels is reported in Table 3.
The concentration of the analyte is considered a vital point for the catalysis process, whereas determining the lower and upper concentration limits can be used to select suitable applications and understand the kinetics of the electrode. Therefore, concentration was studied for different fuels on IN738 electrode. As represented in Fig. 5a-c, the CVs of electrochemical oxidation of ethylene glycol, ethanol, and urea are shown and inset of Fig. 5 shows the linear relation between anodic peak current (at potential of 0.6 V (vs. Ag/AgCl) and fuel concentration (1.0 M). The effect of concentration was studied over concentration ranges (0.025 to 1.0 M). As the result of zero boundary surfaces, the observed anodic current shows two linear regions owing to the electrode's lower sensitivity toward low concentration. Otherwise, the electrode's activity was linear until the concentration reached 1.0 M, indicating a wide range of active species and lower poisoning due to the co-metals inserted in the alloy crystal.
Furthermore, the durability of electrodes was studied using chronoamperometry at oxidation potential (0.55 V vs. Ag/ AgCl) for each fuel. The experiment was carried out in a solution of 1.0 M of (ethylene glycol, ethanol, and urea) and 1.0-M KOH for 5 h of continuous oxidation. As shown in Fig. 6a, the chronoamperogram reflects the high stability of IN738 electrode. Consequently, the activity was decreased by a percentage of 9, 7, and 11% for ethylene glycol, ethanol, and urea, respectively. The electrode's durability was expected due to the alloy's CO tolerance as a result of presence of multitransition metals incorporated in the crystal. Thus, the robust stability of the alloy structure prevents catalyst mass loss.
For constant potential experiments, the current is decreased because of several aspects, like surface poisoning, damage to the electrode surface, and the accumulation of uncompleted  oxidized species [61][62][63]. Accordingly, the expected CO tolerance of IN738 alloy regards the effect of the co-catalyst, where the Ni-based co-catalyst exhibits high resistance toward CO adsorption [64]. Furthermore, using as-received alloy without casting the catalyst on the electrode surface promotes the surface's stability for long-term oxidation.

Kinetics study
The electrooxidation in the basic medium is mainly dependent on Ni(OH) 2 and NiOOH conversion. Therefore, the estimated surface coverage is calculated by cyclic voltammetry (seen Fig. 7a) in the solution containing 1.0-M KOH for various scan rate ranges as follows (Eq. 6) [65]: where J is the oxidation current, n is the electron number, Γ* is the surface coverage, and T is the temperature. Figure 7b displays the linear relation of scan rate vs. oxidation current.
That the studied surface coverage equal to 4.199 × 10 -7 mol. cm −2 , recognization of the electrochemical process and kinetics coefficient must be established. For different fuels, Fig. 7c-e shows that cyclic voltammetry was employed to find out the diffusion coefficient at a wide sweep rates (5 to 1000 mV s −1 ). As fuel electrochemical oxidation is considered an irreversible reaction, Randles-Sevcik's relation was used to determine the diffusion coefficient for different fuels toward the IN738 electrode surface. Thus, the diffusion coefficients are provided by the following Eq. (7) [66]: where I is the current in (A), n is the electron number, n o is the rate-determining step electron, ν is the scan rate, A is electrode area, C is the fuel concentration, and D is the diffusion coefficient. Figure 7f shows the relation between the oxidation current (at 0.7 V vs. Ag/AgCl) versus the square root of scan rate along with a wide range of sweep rate, where the diffusion coefficient is provided for ethylene glycol, ethanol, and urea as 1.038 × 10 -6 , 1.5 × 10 -6 , and 0.64 × 10 -6 cm 2 s −1 , respectively.

EIS
For further information about oxidation on IN738 surface, the electrochemical impedance was measured at a constant potential of 0.5 V (vs. Ag/AgCl) in an alkaline solution holding different fuels (i.e., ethylene glycol, ethanol, and urea). Figure 8 displays Nyquist graph for the mentioned fuels. Whereas, the semi-circuit was observed for the charge transfer electrochemical oxidation process. The resultant EIS data were fitted by NOVA software, as illustrated in Fig. 9. The solution resistance was connected in series with two cells in the fitting circuit. In each cell, double-layer resistance is connected to a constant phase element in a parallel connection. Accordingly, the presence of the constant phase element was reported in the literature as a surface roughness factor [67]. Otherwise, the constant phase elements are considered for different behaviors, such as distribution of reaction rates, surface thickness, homogeneity, and roughness [67][68][69][70].
The estimated EIS parameters are provided in Table 4. It was observed that the impedance of the working electrode in the presence of ethanol is the lowest value among the other fuels. The electrochemical oxidation of ethanol was expected to be easier upon IN738 electrode because of the molecule's lower activation energy and simple structure. The fuel with a higher oxidation current density was found to have lower charge transfer resistance than the other electrodes. Therefore, the faster electron transfer led to higher oxidation activity [71].

Conclusion
IN738 superalloy showed efficient electrooxidation of different fuels, like ethylene glycol, ethanol, and urea. The presence of a high percentage of nickel IN738 alloy enhanced the electrochemical activity of IN738 in the alkaline medium. The activity of IN738 toward ethanol is higher than EG fuel due to ethanol's uncomplicated structure and its higher diffusion rate. IN738 has high catalytic efficiency toward urea electrooxidation because of active elements like Co, Cr, Nb, and Mo that boost the performance of the superalloy surface toward urea electrooxidation. The high structural stability of IN738 enhanced the long-term stability of the electrooxidation process and the decrease in current after 4 h of continuous oxidation approached 10% for most cases.

Conflict of interest
The authors declare that there is no conflict of interest regarding the publication of this paper.
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