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Article

From PET Bottles Waste to N-Doped Graphene as Sustainable Electrocatalyst Support for Direct Liquid Fuel Cells

by
Noha A. Elessawy
1,
Gordana Backović
2,
Janesuda Hirunthanawat
2,
Marta Martins
2,
Lazar Rakočević
3,
Marwa H. Gouda
4,
Arafat Toghan
5,6,
Mohamed E. Youssef
1,
Biljana Šljukić
2 and
Diogo M. F. Santos
2,*
1
Computer Based Engineering Applications Department, Informatics Research Institute IRI, City of Scientific Research and Technological Applications (SRTA-City), Alexandria 21934, Egypt
2
Center of Physics and Engineering of Advanced Materials, Laboratory for Physics of Materials and Emerging Technologies, Chemical Engineering Department, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal
3
Vinča Institute of Nuclear Sciences-National Institute of the Republic of Serbia, University of Belgrade, Mike Petrovića Alasa 12-14, 11000 Belgrade, Serbia
4
Polymer Materials Research Department, Advanced Technology and New Materials Research Institute (ATNMRI), City of Scientific Research and Technological Applications City (SRTA-City), Alexandria 21934, Egypt
5
Chemistry Department, College of Science, Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh 11623, Saudi Arabia
6
Chemistry Department, Faculty of Science, South Valley University, Qena 83523, Egypt
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(3), 525; https://doi.org/10.3390/catal13030525
Submission received: 27 January 2023 / Revised: 25 February 2023 / Accepted: 1 March 2023 / Published: 4 March 2023
(This article belongs to the Special Issue Recent Advances in Energy-Related Materials in Catalysts)
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Abstract

:
Direct liquid fuel cells represent one of the most rapidly emerging energy conversion devices. The main challenge in developing fuel cell devices is finding low-cost and highly active catalysts. In this work, PET bottle waste was transformed into nitrogen-doped graphene (NG) as valuable catalyst support. NG was prepared by a one-pot thermal decomposition process of mineral water waste bottles with urea at 800 °C. Then, NG/Pt electrocatalysts with Pt loadings as low as 0.9 wt.% and 1.8 wt.% were prepared via a simple reduction method in aqueous solution at room temperature. The physical and electrochemical properties of the NG/Pt electrocatalysts are characterized and evaluated for application in direct borohydride peroxide fuel cells (DBPFCs). The results show that NG/Pt catalysts display catalytic activity for borohydride oxidation reaction, particularly the NG/Pt_1, with a number of exchanged electrons of 2.7. Using NG/Pt composite in fuel cells is anticipated to lower prices and boost the usage of electrochemical energy devices. A DBPFC fuel cell using NG/Pt_1 catalyst (1.8 wt.% Pt) in the anode achieved a power density of 75 mW cm−2 at 45 °C. The exceptional performance and economic viability become even more evident when expressed as mass-specific power density, reaching a value as high as 15.8 W mgPt−1.

1. Introduction

Eco-friendly energy storage and conversion devices are severely needed to solve worldwide energy crises. A fuel cell is a device that converts chemical energy into electricity. Hydrogen is primarilyconsidered to be an energy carrier. In the 1970s, liquid hydrogen was used by NASA as a fuel for space shuttles, and more recently it was employed in fuel cell electric vehicles (FCEVs) [1]. Hydrogen is typically used as compressed gas, which has several issues, such as purification, transportation, and storage, which may lead to safety problems for mobile users. Complex hydrides, such as NaBH4, LiH, and NaH, are known to be capable of storing hydrogen, which makes them a possible alternative to solve this problem.
The direct borohydride peroxide fuel cell (DBPFC) gained significant interest as a strong candidate for different portable applications due to simple liquid fuel handling and its high energy density [2,3,4]. In particular, DBPFCs present much higher theoretical voltage (E0 = 3.01 V) than that of the DMFC (E0 = 1.21 V) and of the H2-O2 PEMFC (E0 = 1.23 V). DBPFC systems have other advantages over conventional hydrogen/oxygen fuel cells, such as long cycle life and environmental safety. Their anode reaction is the direct oxidation of sodium borohydride (NaBH4) in sodium hydroxide (NaOH) solution. Theoretically, the electrooxidation of NaBH4 releases eight electrons, as shown in Equation (1). However, due to the unavoidable hydrolysis of borohydride (BH4−) on the electrocatalyst, the number of released electrons is always less than eight [5,6]. The cathodic reaction when hydrogen peroxide is used as an oxidant is provided by Equation (2). Thus, the overall fuel cell reaction occurring in a DBPFC is provided by Equation (3).
BH4− + 8OH− → BO2− + 6H2O + 8e−     E0 = −1.24 V vs. SHE,
H2O2 + 2H+ + 2e− → 2H2O     E0 = 1.77 V vs. SHE,
NaBH4 + 4H2O2 → NaBO2 + 6H2O     E0 = 3.01 V.
Achieving a complete eight-electron transfer from BOR is virtually impossible due to the quasi-spontaneous hydrolysis of BH4− taking place at the electrode surface, generating hydroxyborohydride (BH3OH−). Hence, the actual reaction of BOR is provided by Equation (4), where n refers to the number of exchanged electrons:
BH4− + nOH− → BO2− + (n − 2)H2O + (4−(n/2))H2 + ne−.
The kinetics and mechanism of borohydride oxidation depend on several variables, such as the concentration of NaBH4 [7], the catalyst materials, and temperature [8]. The borohydride oxidation reaction (BOR) was studied in different materials, including noble and non-noble metals, i.e., platinum (Pt) [9], gold (Au) [10], silver (Ag) [10,11], palladium (Pd) [12], and nickel (Ni) [13]. Au was considered for a long time as the most Faradaic-efficient material for BOR, with the advantage of being simultaneously inactive for the competing BH4− hydrolysis reaction, allowing an anodic transfer of close to eight electrons. However, BOR kinetics at Au electrodes are somewhat sluggish. Pd is also electrocatalytically active for both the oxidation of BH4− and its intermediates, as well as for H2 generation/oxidation; however, reaction kinetics are still slower than for Pt [14]. Ni has been reported as a low-cost BOR catalyst ensuring a low (below 0 V vs. RHE) open-circuit potential (OCP) and enabling good net BOR currents [15]. Alloying or mixing different group metals, such as PdNi or CuNi, has increased the activity and selectivity for BOR [16,17,18,19].
Among most typical catalysts, Pt is usually the benchmark material for many electrochemical energy conversion and storage applications, including as an electrocatalyst for BOR, owing to its good performance on the fast exchange of between two and four electrons during BH4− oxidation [20]. Although it also causes the hydrolysis of BH4− [21], many studies have indicated that Pt is one of the most attractive metal catalysts. Along with high performance, Pt comes with a higher price. For this reason, intensive research is underway to develop new highly active catalysts that can use smaller amounts of Pt. Generally, Pt is anchored on low-cost carbon support to reduce the electrocatalyst cost, increase the catalyst surface area, and create abundant active sites that enhance electrocatalyst performance [21,22,23,24].
Graphene (G) has proven its efficiency as an excellent 2D support material due to its superior conductivity and unparalleled layers structure with a very high specific surface area and good chemical and thermal stability [25,26,27]. These qualities led to graphene being used as a support material in energy applications [28,29]. Furthermore, by doping the graphene structure with N atoms, the electron density of the adjacent C atoms is rearranged, prompting an electrophilic center in the neighboring N atoms, and changing the geometry and enhancing the electron donor character of the prepared nitrogen-doped graphene (NG) [30,31].
The present study has been motivated by the need to develop sustainable carbon-supported electrocatalysts with low cost for fuel cell applications. At the same time, removing non-degradable plastic waste accumulating in the world associated with humankind’s excessive plastic use in everyday life is crucial. Considering that plastic is suitable for conversion to carbon material, as it is carbon-rich and the derived carbon contains surface functional groups that enhance its electrochemical performance, efforts are underway toward the valorization of this waste and further use in energy conversion devices [32,33]. Herein, NG is produced by the thermal decomposition of waste PET bottles with urea, and its performance as a stable, high surface area support for Pt nanoparticles is assessed for BOR. The proposed NG synthesis has many advantages over previously followed approaches, including straightforward reaction setup and operational steps, an effective, simple, quick, and environmentally friendly one-pot method that can be easily scaled up, and promising industrial applications. The likelihood of fuel cell commercialization may increase if large-scale, low-cost production is feasible. The electrochemical characterization of the synthesized NG/Pt composite electrocatalysts was performed by cyclic voltammetry (CV) and linear scan voltammetry (LSV), demonstrating the potential application of these materials in DBPFC anodes.

2. Results and Discussion

2.1. Morphological and Structure Characterization of Graphene Doped-N and NG/Pt Catalysts

The surface structure of electrocatalysts greatly influences the catalytic reaction; therefore, the morphology and surface structure of prepared materials were explored using various characteristic tools. From the XRD pattern for prepared NG, NG/Pt_0.5, and NG/Pt_1, as shown in Figure 1a, a slight shifting of the NG/Pt_1 peak corresponding to NG was observed, and that may be due to the introduction of Pt nanoparticles inside NG layers, which alters the interlayer spacing [34]. Moreover, obvious diffraction peaks at 2θ values of 26°, 42.3° and 44.3° are observed, which are indexed to (002), (100), and (101) reflections, respectively, corresponding to the standard C peak (JCPDS No. 75-1621). Additionally, these results indicate that Pt atoms are dispersed uniformly onto NG in the form of small clusters, and no obvious diffraction peaks arising from the possible impurity phases for Pt are observed in X-ray, which may suggest the quantity of Pt used is very small and cannot be detected by XRD [35].
The FTIR spectra of NG, NG/Pt_0.5, and NG/Pt_1 samples are shown in Figure 1b. IR spectra of the three samples exhibit a distinct broad absorption at approximately 3450 cm−1 corresponding to the stretching vibration mode of –OH, and the graphitic nature of both samples can be evidenced by the presence of in-plane C=C vibration at 1680 cm−1, which is an intrinsic characteristic of the sp2 graphitic materials [36,37,38], and the peak at approximately 1680 cm−1 was assigned to the C=O bond. There are no obvious characteristic peaks for Pt.
The adsorption–desorption isotherms of N2 for synthesized samples appear to be characteristic IUPAC-type IV nature, Figure 1c. A hysteresis loop at relative pressure from 0.4 to 1.0 confirmed the mesoporous structure, while a vertical tail near the relative pressure of 1 witnessed the presence of micropores. Table 1 shows the specific surface area with the mean pore diameter for the prepared samples. The distribution of the average pore sizes observed by the BJH method revealed the highest number of pores of large diameter in the case of NG/Pt_1 (Figure 1d). This, in addition to the largest surface area, can provide a large number of active sites in case of NG/Pt_1 and thus boost its catalytic activity.
The morphology of the as-prepared materials was investigated using transmission electron microscopy (TEM). NG/Pt_1 sample (Figure 2a) shows irregular, non-uniform distributed sheets of NG, while Pt nanoparticles appear as dark spots corresponding to semi-spherical particles packed between the NG layers. The size of Pt nanoparticles ranged between 6.6 and 12.2 nm, with the average size of the Pt nanoparticles of 8.6 nm, as measured from TEM images using ImageJ software. Furthermore, the selected area electron diffraction (SAED) pattern was used for further analysis of NG/Pt_1 sample (Figure 2b). The SAED pattern shows dispersed bright spots and honeycomb rings overlay each other, confirming the multilayered and polycrystalline structure of NG/Pt_1. In addition, a comparably weaker spot of Pt adjoining the carbon lattice is noticeable.
The XPS survey spectra of NG, NG/Pt_0.5, and NG/Pt_1 samples are shown in Figure 3a. The main lines of C 1s, O 1s, and N 1s are clearly visible in all three spectra. Additionally, the Pt 4f line is present in two samples doped with Pt, with the Pt 4f line of the NG/Pt_1 sample having higher intensity as expected. High-resolution spectra of C 1s and N 1s for the NG sample are shown in Figure 3b,c, respectively. C 1s spectra can be deconvoluted into five peaks characteristic of graphene [39]. A narrow peak at 285.1 eV, corresponds to sp2 hybridized double carbon bond C=C, while a peak at 284.8 eV corresponds to sp3 hybridized single carbon bond C-C. The peak at 258.8 eV can be assigned to either C-O or C-N bond or a mixture of both signals. The peak at 286.8 eV originates from the double carbon–oxygen bond C=O, and a final peak at 288.6 eV most probably originates from the O=C-OH bond since the FTIR spectroscopy analyses (Figure 1b) indicate the presence of the –OH group. Peaks corresponding to C-O and C-N bonds are almost at the same position and, therefore, cannot be accurately deconvoluted [40,41]. Relatively low peak intensity of oxygen-containing groups indicates that the graphene is highly reduced. Nitrogen N 1s spectrum is deconvoluted into three peaks. Peaks at 398.6 eV, 399.2 eV, and 400.3 eV correspond to pyridinic, pyrrolic, and graphitic nitrogen groups [40,41].
The high-resolution spectra of Pt 4f for NG/Pt_0.5 and NG/Pt_1 samples are shown in Figure 4a,b, respectively. For NG/Pt_0.5 sample, the Pt 4f peak contains only one component shown in a typical doublet. The peak at 75.7 eV corresponds to PtO2 from Pt 4f7/2, while the peak at 79.8 eV corresponds to PtO2 from Pt 4f5/2 [42,43]. Highly upshifted positions of peaks indicate the presence of highly oxidized Pt4+. The peak position shows an even higher upward shift than reported in [43] due to the substrate effect of doped nitrogen into graphene. For NG/Pt_1 sample, Pt 4f line is deconvoluted into two components. The first component at 72.4 eV and 75.9 eV corresponds to PtO from Pt 4f7/2 and Pt 4f5/2 peaks, respectively. The second component at 74.7 eV and 77.9 eV corresponds to PtO2 from Pt 4f7/2 and Pt 4f5/2 peaks, respectively [43,44]. Pt present in this sample is slightly less oxidized, showing the presence of both Pt2+ and Pt4+ species. All four peaks match the positions reported in the literature [43,44], showing only a slight upward shift of less than 0.5 eV in all cases. That indicates a slightly lower impact of the substrate on electron binding energy with an increased amount of Pt.

2.2. Electrochemical Measurements

The BOR catalytic performance of the support and the two NG/Pt electrocatalysts prepared was initially investigated by scanning comparative CVs in 2 M NaOH with 0.03 M NaBH4 and in pure 2 M NaOH solution. The CVs of NG/Pt catalysts depicted in Figure 5a,b show the absence of anodic peaks in the BOR potential region in NaOH solution, implying that the current generated during CV measurements run in the presence of BH4− originated from its oxidation. Moreover, in the scanned potential range, CVs of NG/Pt catalysts showed one oxidation peak (ca. 0.6 V and 0.7 V for NG/Pt_1 and NG/Pt_0.5, respectively) in the positive scan. An additional sharp oxidation peak was evident in the reverse scan, more pronounced for NG/Pt_1. This peak can be attributed to the oxidation of byproducts originating from the BH4− hydrolysis reaction, such as oxidation of adsorbed BH3OH− formed during the anodic scan and staying adsorbed on the oxidized electrode surface until being reactivated by reduction in the surface oxides. BH4− electrooxidation is a complex reaction with several oxidation peaks: a sharp oxidation peak (a1) around 0.9 V and a broad oxidation hump (a2) around 1.4 V on the direct scan, followed by a well-defined oxidation peak at 1.15 V on the reverse scan. Peak a1 is attributed to the direct oxidation of BH4− as an eight-electron process, while wave a2 and peak c1 are attributed to the oxidation of BH3OH− generated as an intermediate during BH4− oxidation. Oxidation peak at reverse scan appears in the potential region where Pt oxides are reduced and the electrode surface is reactivated [45]. Thus, there is a clear competition between the BH4− oxidation and its catalytic hydrolysis in the case of NG/Pt anode catalyst, Scheme 1. H2 molecules generated during hydrolysis may leave the catalyst’s surface or be further oxidized on the same (or another) Pt site [8,10,46].
NG/Pt_1 exhibited higher BOR catalytic performance, reaching the current density of ca. 4.5 mA cm−2. The enhanced catalytic activity of NG/Pt_1 partially originates from the higher Pt loading. Still, it should be kept in mind that the Pt loading is rather low in the case of both tested electrocatalysts (0.9 wt.% Pt and 1.8 wt.% Pt for NG/Pt_0.5 and NG/Pt_1, respectively). However, as XPS analysis revealed, NG/Pt_1 sample is less oxidized and it is found in both PtO and PtO2 states. On the other hand, NG/Pt_0.5 sample is more oxidized and is found mostly in PtO2 state, with Pt in oxidized form being less active for direct BH4− oxidation. Moreover, the specific surface area of NG/Pt_1 (303 m2 g−1) was notably higher than that of NG/Pt_0.5 (181 m2 g−1), providing a higher number of active sites for BOR to occur. Both features are responsible for a better performance of NG/Pt_1 compared to NG/Pt_0.5, with the latter exhibiting an apparent capacitive behavior in NaOH solution. It should be mentioned that the N-doped graphene support was also tested for BOR but did not show activity.
The reaction kinetics were additionally examined using RDE LSVs (Figure 6). The corresponding j−1 vs. ω−1/2 plots for the currents taken at 0.7 V are shown as inset of Figure 6b. From the slope of j−1 vs. ω−1/2 plots and using the Koutecky–Levich equation, n values were obtained. n values of 2.7 and 0.7 were obtained for NG/Pt_1 and NG/Pt_0.5, respectively. The value determined for NG/Pt_1 is comparable with typical n values reported in the literature for BOR at Pt or Pt alloys (two to six electrons) [47].
Poisoning of Pt-based electrocatalysts during BH4− oxidation has been reported to be pronounced, especially in BH4− solutions of higher concentration [48,49]. Still, no activity loss was observed during BOR experiments within this study. This suggests that using the N-doped graphene support stabilizes Pt nanoparticles, ensuring their uniform distribution throughout the catalyst’s operation. The Pt active centers are cleaned of the adsorbed intermediates upon the intermediates’ further oxidation.
To evaluate the performance of an NG/Pt_1 anode for a DBPFC, fuel cell tests were performed using a Pt mesh cathode and an NG/Pt_1 anode. Figure 7 depicts the polarization and power density curves of a single cell with NG/Pt_1 anode operating at three different temperatures, 25, 35, and 45 °C.
As expected, at a higher temperature, the kinetics of the fuel cell reactions were faster, leading to a significant improvement in cell performance. At 45 °C, the power density value reached a maximum of 75 mW cm−2 at 1.8 V. The exceptional performance of NG/Pt_1 and great economic viability become even more evident when expressed as mass-specific power density. Namely, a mass-specific peak power density value as high as 15.8 W mgPt−1 was reached at a cell voltage value of 1.7 V and a current density of 48.2 A mgPt−1. Obtained DBPFC results were compared with those reported in the literature [49,50,51,52]. Table 2 summarizes the DBPFCs results involving Pt and different carbon supports. Notably, the mass-specific performance of DBPFC with NG/Pt_1 clearly surpasses the ones reported in the literature. High mass-specific power density evidences the benefits of using the herein-prepared N-doped graphene as a carbon support that facilitates charge transfer. Its high specific surface area promotes fast charge transfer, enhancing the conductivity path and thus leading to high electrical conductivity [8]. The high specific surface area comes from a higher inter-layer distance (or d-spacing). It has been shown that BOR kinetics and its completion are influenced to a great extent by the mass transport of reactants and intermediates to/from the electrode material’s active sites [53,54]. The large pores and layered structure of the herein-prepared NG facilitate both the transport of active species within the electrocatalyst’s porous structure to active sites and the escape of generated hydrogen (thus cleaning the active sites from the gas bubbles).

3. Materials and Methods

3.1. Preparation

Nitrogen-doped graphene was synthesized from PET plastic water bottle wastes by pyrolyzing them with urea at 800 °C for 1 h, following a procedure similar to that reported by Elessawy et al. [34]. The final resulting powder was collected and ground as a fine powder. NG/Pt catalysts with metal loadings of 1.8 and 0.9 wt.% Pt were prepared by adding the required amounts of NG powder and chloroplatinic acid solution (H2PtCl6·8 wt.%H2O) into a beaker under gentle stirring for 1 h. Then, 1 mL of 1 M NaBH4/1 M NaOH solution was added with gentle stirring for 24 h. The resultant catalyst was filtered and rinsed with de-ionized water until no Cl was identified, then dried for 12 h at 60 °C. All chemicals were acquired from Sigma-Aldrich (St. Louis (MO), USA) and used as received, unless otherwise specified.

3.2. Characterization

The basic characterization analysis for prepared catalysts was carried out, including determining the crystal size from XRD (Shimadzu-7000, Kyoto, Japan) with CuKα beam and a scan speed of 4° per min, the surface functional groups by using FTIR spectrophotometer (Shimadzu FTIR-8400S, Kyoto, Japan) with spectra range from 400 to 4000 cm−1. The elemental analysis of prepared catalysts was measured by using EDAX, and morphology was assessed by TEM (JEOL JEM-1230, Tokyo, Japan). The composition of samples was examined by XPS (XP50M X-ray source for Focus 500 and PHOIBOS 100/150 analyzer with AlKα source (1486.74 eV) at a 12.5 kV and 32 mA) and data were analyzed using SPECS Systems (SPECS GmbH, Berlin Germany). The average pore volume and surface area were obtained using BET and BJH adsorption techniques.

3.3. Electrochemical Measurements

The catalytic ink was prepared by ultrasonically dispersing the electrocatalyst (5 mg) into a polyvinylidene difluoride (PVDF) in N-methyl pyrrolidone (NMP) solution (5 wt.%, 125 µL) for 30 min. The working electrode was prepared by depositing catalyst ink (3 µL) onto a glassy carbon electrode surface. The working electrode was left in the oven for drying at 75 °C overnight. A typical three-electrode setup was used in the experiment. The platinum coil was used as a counter electrode and a saturated calomel electrode (SCE, Hanna Instruments, Woonsocket (RI), USA) was used as a reference electrode.
To examine the catalysts behavior in 2 M NaOH at room temperature, 2 CVs were recorded in the range from –1.1 V to 0 V at 10 mV s−1. To study the BOR at prepared catalysts at room temperature, 0.03 M of NaBH4 in 2 M NaOH solution was prepared immediately before the measurements. Three cycles were run from the open circuit potential (OCP) to 0 V at 10 mV s−1. Linear scan voltammetry with a rotating disk electrode was run in the range from OCP to 0 V at 10 mV s−1 and different rotation rates between 0 and 2400 rpm. Finally, fuel cell tests were conducted under the optimized conditions described in our previous study using 1 M NaBH4 in 4 M NaOH and 5 M H2O2 in 1.5 M HCl solutions as an anolyte and catholyte, respectively [51].

4. Conclusions

PET bottle waste was successfully valorized into N-doped graphene as electrocatalyst support. Subsequently, NG/Pt electrocatalysts containing only 0.9 wt.% Pt and 1.8 wt.% Pt were prepared and characterized by XRD, FTIR, XPS, and TEM. N2-sorption analysis revealed a notably higher specific surface area of NG/Pt_1 that provides a higher number of active sites, thus contributing to its higher activity.
The results obtained from CV and LSV measurements revealed that the catalysts performed well towards BOR. The NG/Pt_1 catalyst exhibited higher activity, evidenced by a number of exchanged electrons of ca. 2.7. Moreover, DBPFC operating at three different temperatures (25, 35, and 45 °C) employing an NG/Pt_1 catalyst showed a maximum power density of 45 mW cm−2 at 25 °C, with a significant improvement with a temperature increase and peak power density of 75 mW cm−2 at 45 °C. Moreover, a mass-specific power density as high as 15.8 W mgPt−1 was reached. Considering the economic aspects of these catalysts and the obtained results, it can be concluded that NG/Pt catalysts are very promising materials for application as anode catalysts in DBPFCs.

Author Contributions

Conceptualization, N.A.E. and D.M.F.S.; methodology, N.A.E., B.Š. and D.M.F.S.; validation, A.T., B.Š. and D.M.F.S.; formal analysis, G.B., M.M.; investigation, N.A.E., G.B., J.H. and L.R.; writing—original draft preparation, N.A.E., M.H.G. and D.M.F.S.; writing—review and editing, M.M., B.Š. and D.M.F.S.; visualization, M.E.Y., A.T. and D.M.F.S.; supervision, B.Š., D.M.F.S.; project administration, N.A.E., M.H.G., M.E.Y. and D.M.F.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Fundação para a Ciência e a Tecnologia (FCT, Portugal) through project EXPL/EQU-EQU/0517/2021. FCT also funded a research contract in the scope of programmatic funding UIDP/04540/2020 (D.M.F. Santos) and contract no. IST-ID/156-2018 (B. Šljukić). Thanks to Academy of Scientific Research and Technology and Bibliotheca Alexandria (ASRT-BA) for supporting this research through post-doctoral research; grant project No. 1436.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

Thanks to IAESTE program for supporting the internship of J. Hirunthanawat.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 13 00525 g001 550
Figure 1. (a) XRD patterns, (b) FTIR spectra, (c) N2 adsorption–desorption isotherms, and (d) average diameter distribution of NG, NG/Pt_0.5, and NG/Pt_1 samples.
Figure 1. (a) XRD patterns, (b) FTIR spectra, (c) N2 adsorption–desorption isotherms, and (d) average diameter distribution of NG, NG/Pt_0.5, and NG/Pt_1 samples.
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Figure 2. (a) TEM image and (b) selected area electron diffraction pattern of NG/Pt_1 sample.
Figure 2. (a) TEM image and (b) selected area electron diffraction pattern of NG/Pt_1 sample.
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Figure 3. (a) XPS survey spectra of NG, NG/Pt_0.5, and NG/Pt_1 samples and high-resolution XPS spectrum of (b) C 1s and (c) N 1s for NG sample.
Figure 3. (a) XPS survey spectra of NG, NG/Pt_0.5, and NG/Pt_1 samples and high-resolution XPS spectrum of (b) C 1s and (c) N 1s for NG sample.
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Figure 4. High-resolution XPS spectra of Pt 4f for (a) NG/Pt_0.5 and (b) NG/Pt_1 samples.
Figure 4. High-resolution XPS spectra of Pt 4f for (a) NG/Pt_0.5 and (b) NG/Pt_1 samples.
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Figure 5. CVs of NG/Pt_1 (a) and NG/Pt_0.5 (b) in 2 M NaOH solution with and without 0.03 M NaBH4 at 10 mV s−1 and room temperature.
Figure 5. CVs of NG/Pt_1 (a) and NG/Pt_0.5 (b) in 2 M NaOH solution with and without 0.03 M NaBH4 at 10 mV s−1 and room temperature.
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Scheme 1. Schematic illustration of the mechanism of BH4− electrooxidation and its hydrolysis as a competing reaction.
Scheme 1. Schematic illustration of the mechanism of BH4− electrooxidation and its hydrolysis as a competing reaction.
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Figure 6. Effect of electrode rotation rate on BOR at NG/Pt_1 (a) and NG/Pt_0.5 (b) with KL plots in the inset. LSVs run at 10 mV s−1 and room temperature.
Figure 6. Effect of electrode rotation rate on BOR at NG/Pt_1 (a) and NG/Pt_0.5 (b) with KL plots in the inset. LSVs run at 10 mV s−1 and room temperature.
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Figure 7. Polarization and power density curves at 25–45 °C for a DBPFC using NG/Pt_1 anode.
Figure 7. Polarization and power density curves at 25–45 °C for a DBPFC using NG/Pt_1 anode.
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Table
Table 1. BET analysis of prepared NG, NG/Pt_0.5, and NG/Pt_1 samples.
Table 1. BET analysis of prepared NG, NG/Pt_0.5, and NG/Pt_1 samples.
Surface Area (BET) (m2 g−1)Mean Pore Diameter (nm)
NG1873.17
NG/Pt_0.5
NG/Pt_1		181
303		3.54
4.75
Table
Table 2. Summary of DBPFCs results employing carbon-supported Pt-based electrocatalysts.
Table 2. Summary of DBPFCs results employing carbon-supported Pt-based electrocatalysts.
AnodeSeparatorFuelOxidantT/°CPmax/W mgPt−1jmax/mAcm−2Source
Pt/PPy2-C12% (0.04 mgPt cm−2)Nafion 1171 M NaBH4 + 4 M NaOH5 M H2O2 + 1.5 M HClRT10.5770.3[8]
Pt/PPy2-C20% (0.04 mgPt cm−2)Nafion 1171M NaBH4 + 4 M NaOH5 M H2O2 + 1.5 M HClRT10.7576.0[8]
Pt/PPy2-C35% (0.06 mgPt cm−2)Nafion 1171 M NaBH4 + 4 M NaOH5 M H2O2 + 1.5 M HClRT10.79120[8]
Pt/NPC3 (4 mgPt cm−2)Nafion 1171 M NaBH4 + 3 M NaOH2 M H2O2 + 0.5 M H2SO4RT10.01120[10]
Pt/CB4 (0.5 mgPt cm−2)Nafion 1170.1 M NaBH4 + 6 M NaOH0.1 M H2O2RT10.0118.0[47]
PtNi/C (1 mgPtNi cm−2)Nafion 1171 M NaBH4 + 2 M NaOH2 M H2O2 + 0.5 M H2SO4450.57100[46]
NG/Pt_1 (0.02 mgPt cm−2)Nafion 1171 M NaBH4 + 4 M NaOH5 M H2O2 + 1.5 M HCl4515.8137This study
RT1—room temperature; PPy2—polypyrrole; NPC3—nanoporous carbon; CB4—carbon black.
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Elessawy, N.A.; Backović, G.; Hirunthanawat, J.; Martins, M.; Rakočević, L.; Gouda, M.H.; Toghan, A.; Youssef, M.E.; Šljukić, B.; Santos, D.M.F. From PET Bottles Waste to N-Doped Graphene as Sustainable Electrocatalyst Support for Direct Liquid Fuel Cells. Catalysts 2023, 13, 525. https://doi.org/10.3390/catal13030525

AMA Style

Elessawy NA, Backović G, Hirunthanawat J, Martins M, Rakočević L, Gouda MH, Toghan A, Youssef ME, Šljukić B, Santos DMF. From PET Bottles Waste to N-Doped Graphene as Sustainable Electrocatalyst Support for Direct Liquid Fuel Cells. Catalysts. 2023; 13(3):525. https://doi.org/10.3390/catal13030525

Chicago/Turabian Style

Elessawy, Noha A., Gordana Backović, Janesuda Hirunthanawat, Marta Martins, Lazar Rakočević, Marwa H. Gouda, Arafat Toghan, Mohamed E. Youssef, Biljana Šljukić, and Diogo M. F. Santos. 2023. "From PET Bottles Waste to N-Doped Graphene as Sustainable Electrocatalyst Support for Direct Liquid Fuel Cells" Catalysts 13, no. 3: 525. https://doi.org/10.3390/catal13030525

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