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Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0
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Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation Nasser A.M. Barakat a,b,c,*, Mohamed A. Yassin a,b, Ahmed S. Yasin a,b, Saeed Al-Meer d a
Organic Materials and Fiber Engineering Department, Chonbuk National University, Jeonju 561-756, Republic of Korea b Bionanosystem Engineering Department, Chonbuk National University, Jeonju 561-756, Republic of Korea c Chemical Engineering Department, Faculty of Engineering, Minia University, El-Minia, Egypt d Central Laboratory Unit, Qatar University, P. O. Box: 2713, Doha, Qatar
article info
abstract
Article history:
Overall, boosting the formation of the active NiOOH layer distinctly improves the electro
Received 22 March 2017
catalytic activity of nickel-based materials. Moreover, due to the adsorption capacity, the
Received in revised form
carbonaceous supports improve the electrocatalytic activities of several function materials
6 July 2017
including nickel. In this manuscript, nitrogen doping is introduced as an effective strategy
Accepted 7 July 2017
to enhance formation of the NiOOH active layer on the surface of nickel nanoparticles
Available online xxx
decorating carbon nanofibers. Typically, addition of urea to the nickel acetate/poly(vinyl alcohol)/water sol-gel leads to obtain nitrogen-doped and nickel-decorated carbon nano-
Keywords:
fibers after electrospinning the prepared solution and calcination the nanofiber mats under
N-doped nanofibers
Ar at 750 C. The invoked characterizations including XRD, TEM, SEM and XPS affirmed
Urea electrooxidation
formation of smooth nitrogen-doped carbon nanofibers decorated by crystalline nickel
Nickel-decorated carbon nanofibers
nanoparticles. Nitrogen doping strongly enhanced the formation of NiOOH active layer on
Electrospinning
the surface of the metallic nanoparticles which distinctly improved the electrocatalytic activity toward urea oxidation. However, the electrochemical measurements indicated that the content of the urea in the initial electrospun solution should be optimized as the results showed that the best nickel surface activation as well as the maximum observed current density can be achieved when the urea content is kept at 5 wt%. Typically, the observed current densities using 1.0 M urea (in 1 M KOH) were 3.15, 29.7, 13.75, 47.55, 14.13 and 9.85 mA/cm2 for the nanofibers obtained from electrospun solutions having 0, 1, 2, 5, 7 and 9 wt% urea, respectively. Moreover, the proposed nitrogen doping strategy leads to perform a distinguish decrease in the onset potential of the urea electrooxidation which reduces the required energy for the electrolysis process. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Organic Materials and Fiber Engineering Department, Chonbuk National University, Jeonju 561-756, Republic of Korea. E-mail address: [email protected] (N.A.M. Barakat). http://dx.doi.org/10.1016/j.ijhydene.2017.07.076 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076
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Introduction Urea-contaminated water creates a serious problem as it is produced in large amounts from the fertilizer industries. It is known that production of 1 ton of the urea fertilizer leads to discharge 0.5 ton of the urea-polluted water. Moreover, human and animal urines can be considered continuous and big sources of the urea-containing wastewater. Urea is not directly toxic, however its hydrolysis into ammonia results in toxicity of both marine and animal life. The ammonium can be oxidized by two groups of bacteria (Nitrobacter and Nitrosomonas), in a process called nitrification, to NO3 with an unstable intermediate product (NO2) [1]. Denitrification takes place under oxygen-shortage media such as wetlands where NO3 is reduced to various gaseous products. Moreover, urea contamination can trigger algae of the oceans to produce domoic acid; a deadly toxin [2]. In contrast to other water pollutants, urea can be exploited as a rich source of hydrogen. Typically, hydrogen content in urea is around 6.67 wt%. In other words, the urea-polluted wastewaters can be theorized as a green and renewable source of energy if the urea content could be decomposed to hydrogen and other environmentally safe gaseous byproducts (CO2 and N2) which also interprets a simultaneous purification. Moreover, compared with other liquid/gas carriers of the hydrogen, urea possesses a variety of distinguished properties such as non-toxic, stable, renewable and non-flammable [3e5]. Electrolysis is an effective and cheap methodology to extract the embedded hydrogen from urea. This new strategy
was proposed to convert urea to hydrogen directly through an electrochemical oxidation based on these reactions [6e9]: Anode : Cathode : Overall :
COðNH2 Þ2 þ 6OH /N2 þ 5H2 O þ CO2 þ 6e 6H2 O þ 6e/3H2 þ 6OH COðNH2 Þ2 þ H2 O/N2 þ 3H2 þ CO2
(1) (2) (3)
Paradoxically, precious metals show an excellent catalytic performance in the electrooxidation of several alcohols, but poor activity has been observed with urea [10]. On the other hand, nickel-based materials, which have lower electrocatalytic activity toward the alcohols compared to the precious metals, revealed higher performance toward urea electrooxidation [11e13]. The main reason behind the precedence of nickel-based materials over Pt toward urea electrooxidation is the associated NiOOH layer which can be generated on the surface by successive sweeping in an alkaline medium. On the other hand, the catalytic power of the precious metals comes from their surface electronic structure. In more details, in contrast to alcohols which require only water molecules to achieve a complete oxidation (e.g. methanol; CH3 OH þ H2 O ¼ CO2 þ 6Hþ þ 6e), urea needs OH group to be oxidized as shown in the aforementioned anode reaction (Eq. (1)) [14]. Therefore, NiOOH is considered a reactant in the urea oxidation reaction [14]. Accordingly, it can be claimed that enhancement of NiOOH formation can strongly improves urea electrooxidation. Likened alcohols oxidation reactions, immobilization of the functional materials on carbonaceous supports distinctly
Fig. 1 e SEM images for the (A) electrospun NiAc/PVA nanofibers, and the nanofibers obtained from calcination of electrospun solutions having (B) 0%, (C) 1%, and (D) 2% urea. Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076
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introduced catalyst has featured activity toward urea electrooxidation due to boosting NiOOH formation, however the content of urea in the original electrospun solution should be optimized.
Experimental details Materials Nickel (II) acetate tetrahydrate (NiAc, 98%, SamChun Chemicals Co Ltd., South Korea), urea (99%, SamChun Chemicals Co Ltd., South Korea), and poly(vinyl alcohol) (PVA; MW ¼ 89,000 g/mol, DC-Chemical Co., Republic of Korea) were used as received without further modifications.
Experimental work Fig. 2 e XRD pattern for the powder obtained from calcination of NiAc/PVA electrospun nanofibers under argon atmosphere at 750 C.
enhances the anodic reaction due to the adsorption capacity [15e18]. Compared to other nanostructures, the large axial ratio gives the nanofibrous morphology good advantage in improvement the electrocatalytic activity [19e21]. From various reported nanofibers making processes, electrospinning attracted the maximum attention due to its high yield, simplicity, applicability and durability [22,23]. Our previous studies indicated that presence of transition metals (especially nickel and cobalt) strongly enhances graphitization of the poly(vinyl alcohol) (PVA) and overcomes the normal full decomposition at low temperatures due to formation of low molecular weight compounds [24]. Therefore, with an improved graphitization, PVA can be exploited as a carbon-rich and cheap precursor for fabrication of the carbon nanofibers; carbon content in PVA is ~48 wt%. This study introduces nitrogen doping as a novel and efficient strategy to enhance formation of the NiOOH layer for nickel nanoparticles-incorporated carbon nanofibers. The proposed electrocatalyst was prepared by electrospinning of urea/poly(vinyl alcohol)/nickel acetate aqueous sol-gel followed by sintering under Ar atmosphere at high temperature; 750 C. The electrochemical measurements revealed that the
First, a stock of NiAc (20 wt%) aqueous solution was prepared. To study the influence of urea content, 0, 0.05, 0.1, 0.25, 0.35 and 0.45 g of urea was dissolved in 5 g of the prepared NiAc solution. Then, each urea/NiAc solution was mixed with 15 g of an aqueous PVA (10 wt%) solution. Typically, the utilized urea amounts were estimated to get final electrospun solutions having 0, 1, 2, 5, 7 and 9 wt% urea. The solutions were vigorously stirred at 50 C for 5 h. The electrospinning process was achieved in a simple set-up at 20 kV with a 15-cm distance between the needle of a syringe containing the solution and the nanofibers collector. The obtained electrospun mats were vacuously dried at 80 C overnight. The calcination process was done in a tube furnace at 750 C with holding time of 5 h under Ar atmosphere.
Characterization The chemical structure of the produced nanofibers was investigated by using Rigaku X-ray diffractometer (XRD, Rigaku, Japan) with Cu Ka (l ¼ 1.540 A) radiation over Bragg angle ranging from 10 to 100 . The nanofibrous morphology was studied by SEM (scanning-electron-microscope; JEOLJSM-5900, Japan). High resolution image was obtained with transmission electron microscope (TEM, JEOL JEM-2010, Japan) operated at 200 kV. X-ray Photoelectron Spectroscopy analysis (XPS, AXIS-NOVA, Kratos analytical Ltd., UK) was used to study the surface chemistry of the studied nanofibers. Cyclic
Fig. 3 e (A) Normal and (B) high resolution TEM images for the NieC nanofibers. Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076
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Results and discussion
Fig. 4 e XPS spectrum for the nanofibers obtained from calcination of (A) NiAc/PVA e 1 wt% urea and (B) NiAc/ PVA e 5 wt% urea electrospun nanofibers under argon atmosphere at 750 C. The inset in panel (A) displays the N1S peak with the background.
voltammetry analyses were performed in three electrodes cell controlled by VersaStat4 instrument. Generally, the urea was dissolved in 1 M KOH solution and the sweep potential range was kept to start from 0.2 to 1.0 V [vs. Ag/AgCl]. Conventional three electrodes cell was used in which platinum wire, glassy carbon and Ag/AgCl electrodes were used as counter, working and reference electrode, respectively. The current density was normalized to the working electrode active area (0.073 cm2). The working electrode was prepared by adding 0.002 g of the prepared nanofibers to a solution consisting of 5 mL Nafion solution (5 wt%) and 400 mL isopropanol. 15 mL of the prepared slurry was poured on the active area of the working electrode. Then, the electrode was dried at 80 C. The mass of the functional material on the surface of the working electrode was determined as 0.073 mg.
Electrospinning shows distinct performance in production of the polymer nanofibers either from solutions or melts. In case of inorganic nanofibers, the utilized metal precursor and polymer have strong impact on the nanofibrous morphology of the final product. Generally, metallic precursors having high polycondensation characteristic reveal good nanofibers. Accordingly, the alkoxides are the most widely used precursors. However, recently, acetates proved that the relatively simple polycondensation feature compared to alkoxides can also provide good nanofibrous morphology. Accordingly, several metals acetates could be utilized to produce good morphology inorganic nanofibers [25]. In this study, using nickel acetate with poly(vinyl alcohol) reveals good electrospun nanofibers as shown in the SEM image (Fig. 1A). Moreover, addition of urea (up to the used 9 wt%) did not affect the obtained nanofibrous morphology (data are not shown). Due to the aforementioned polycondensation tendency of the nickel acetate, after the calcination process, there is no strong influence on the nanofibrous morphology as shown in Fig. 1B which displays the nanofibers produced from calcination of electrospun nanofibers obtained from urea-free NiAc/PVA solution. Interestingly, as in the case of the electrospun nanofibers, addition of urea did not strongly affect the nanofibrous morphology after the calcination process as shown in Fig. 1C and D. However, crosslinking was observed among some nanofibers as shown in the figure. It is noteworthy mentioning that the crosslinking was also observed in the other formulations which are not presented in the figure. XRD is highly trustable analytical technique to investigate the chemical composition of the inorganic materials. As shown in Fig. 2, the obtained XRD pattern confirms formation of pure Ni and carbon. Typically, the standard diffraction peaks of nickel appear clearly at 2q of 44.5 , 51.8 , 76.4 and 92.9 corresponding to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) crystal plans, respectively (PDF# 04-0850). The broad peak at 2q of A 26.3 corresponding to an experimental d spacing of 3.37 reveals presence of graphite-like carbon (d (002), PDF#; 411487). It is noteworthy mentioning that similar XRD patterns were obtained with all formulations. Fig. 3A demonstrates normal TEM image for the prepared nanofibers (N-free). As shown, numerous crystalline nanoparticles are distributed along with the amorphous nanofiber matrix. The crystallinity of the observed nanoparticles could be determined by high resolution TEM image; Fig. 3B. The image refers to high crystallinity due to clear appearance of the parallel crystal planes. Moreover, as shown in the inset, the distance between two successive planes matches the standard value of the pristine nickel metal. On the other hand, the amorphous nanofiber matrix can be assigned to the detected graphite in the XRD pattern. Under the utilized high calcination temperature, it is expected that urea has been decomposed to nitrogen and other compounds. Presence of nitrogen in the introduced nanofibers was investigated by XPS analysis. As shown in Fig. 4A and B which display the XPS spectra of the nanofibers obtained from electrospun solutions containing 1 and 5 wt%, respectively, the observed peak at a binding energy of ~399 eV representing
Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076
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Fig. 5 e Surface activation by cyclic voltammetry process for the nanofibers obtained from electrospun solutions having (A) 0 wt%, (B) 5 wt%, (C) 7 wt% and (D) 9 wt% urea in presence of 1 M KOH at scan rate of 100 mV/s and 25 C.
the N1S electron confirms presence of nitrogen in the investigated samples [26]. The inset in Fig. 4A demonstrates the N1S peak with the background. It is known that XPS is a surface analyzing technique. In more details, XPS can detect the elemental composition within a 10e12 nm surface layer. Therefore, XPS was invoked as qualitative analysis technique and its quantitative analyses results cannot be exploited as a real representative for the nitrogen content of the whole materials. However, based on the XPS results, the nitrogen content in the nanofibers obtained from the electrospun solution having 5 wt% urea is 2.2 At%. Overall, based on the utilized physicochemical characterizations, it can be concluded that the introduced nanofibers are nitrogen-doped and nickel-decorated carbon nanofibers. As it was aforementioned, surface activation can be achieved by sweeping within a proper potential window in an alkaline medium. As examples, the cyclic voltammetric behaviors of four prepared nanofibers are shown in Fig. 5.
Polarization was started by a potential scanning at a scan rate of 100 mV/s from 1000 mV to 200 mV (vs. Ag/AgCl). Overall, for the nickel-based materials, there are two peaks are observed in the activation voltammogram; the first one appears at the negative potential is attributed to formation of nickel hydroxide [10,21,27], the first peak cannot be observed in Fig. 5 as it is beyond the used potential window: Ni þ 2OH 4NiðOHÞ2 þ 2e
(4)
The current densities of the redox peaks of this transformation are usually small from the beginning cycle and then the peaks could not be distinguished in the subsequent cycles [27e29]. The second peaks region is observed at the positive potential and related to the oxidation of Ni(OH)2 to NiOOH. The representing peaks are always strong and the reaction can be explained by this equation [29e31]: NiðOHÞ2 þ OH 4NiOOH þ H2 O þ e
(5)
Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076
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Fig. 6 e (A) 10th cycle in the surface activation voltammograms for all the prepared nanofibers in presence of 1 M KOH at scan rate of 100 mV/s and 25 C, and (B) influence of the urea content on the current density of the anodic peak.
point to small formation for the NiOOH active layer. On the other hand, in case of addition 5 wt% urea to the electrospun solution (Fig. 5B), the peak started at a high current density which is distinctly enhanced with increasing the number of cycles. Further addition of urea results in enhancing the peak current as shown in Fig. 5C and D which display the activation voltammograms of the nanofibers obtained from electrospun mats having 7 and 9 wt% urea, respectively. However, the peak maximum current density for both formulations is less than that in case of 5 wt% urea sample (Fig. 5B). Fig. 6A shows 10th cycle in the activation voltammograms for the all prepared nanofibers. As shown in Fig. 6B, the nanofibers obtained from electrospun solution having 5 wt% urea reveal the maximum anodic current density for the Ni(OH)2/NiOOH transformations. The NiOOH surface coverage (G) in mol/cm2 can be determined from this equation [32]. Fig. 7 e Influence of urea content on the surface coverage of the prepared nanofibers. Observable increase of the peak current at the cathodic reaction can be noticed with increasing the number of sweeping cycles which indicates enhancing the entry of OH into the Ni(OH)2 surface layer. Consequently, progressive formation of a thick layer from NiOOH can be performed [27]. Therefore, the observed redox peaks in Fig. 5 can be assigned to Ni(OH)2/NiOOH transformation process. However, as shown in the figure, the urea content in the original electrospun solution has a distinct influence on the peaks current density and its improvement upon increasing the cycle number. In more details, as shown in Fig. 5A which displays the voltammograms of the nanofibers obtained from urea-free electrospun solution, there is a very little increase in the peak current density with increasing the number of the CV cycle. Moreover, the corresponding current density is relatively small compared to the other formulations. These findings
IP ¼
2 2 n F yAG 4RT
(6)
where Ip is the peak current, n is the number of electrons sharing in the reaction, v is the scan rate, T is the temperature in Kelvin, R is the universal gas constant, F is the Faraday's constant, and A is the electrode area. Fig. 7 shows a histogram the NiOOH surface coverage for the prepared nanofibers. As shown in the histogram, addition of urea to the electrospun solution enhances NiOOH formation up to 7 wt%. However, more increase in the urea content (i.e. 9 wt%) has a negative impact on the surface activation. Interestingly, compared to nitrogen-free nanofibers, addition of 5 wt% urea to the electrospun solution leads to increase the amount of the formed NiOOH four times which reflected distinct improvement in the urea electrocatalytic activity as it will be discussed later. As it is known that the urea concentration can affect the electrooxidation reaction. Fig. 8 demonstrates the influence of urea solution concentration (in 1 M KOH) in case of nanofibers prepared from different electrospun solutions based on the
Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076
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Fig. 8 e Electrocatalytic activity at different urea concentrations for the nanofibers obtained from electrospun solutions having (A) 0%, (B) 1%, (C) 2%, (D) 5%, (E) 7% and (F) 9% urea in presence of 1 M KOH at scan rate of 50 mV/s and 25 C.
Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076
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Fig. 9 e Electrocatalytic activities for all the prepared nanofibers in presence of (A) 0.5 M, (B) 1.0 M, (C) 3.0 M and (D) and 4.0 M urea (in 1 M KOH at scan rate of 100 mV/s and 25 C).
urea content. Overall, based on the reaction stoichiometry, low urea concentration reveals high catalytic activity. This hypothesis is confirmed with most of the synthesized nanofibers as displayed in the figure. Interestingly, the results also indicated that there is a distinct influence of the content of the nitrogen in the prepared nanofibers on the urea electrooxidation process. Moreover, it can be observed that the general trend in the obtained results resembles to the case of the activation process (Figs. 5 and 7). In more details, the sample owing the best activation process (5 wt% urea sample) shows the maximum electrocatalytic activity toward urea oxidation (Fig. 8D). Furthermore, nitrogen doping leads to have a clear appearance for the urea oxidation peak as shown in all nitrogen-contained nanofibers. Urea oxidation peak is very clear at the low nitrogen content nanofiber (sample obtained from the electrospun solution having 1% urea; Fig. 8B), and also at the maximum utilized urea content (9 wt%; Fig. 8F). Moreover, the urea oxidation peak shifted to negative direction after nitrogen doping for all formulations.
Theoretically, urea electrolysis reaction requires relatively high energy due to the considerable low cathode reaction potential (water reduction reaction (Eq. (2))). Accordingly, owing low onset potential is an important characteristic for the anode material as it leads to decrease the required power. In this regard, decreasing the onset potential upon nitrogen doping which is observed in Fig. 8 (marked by the arrows) is considered another important advantage. Numerically, the observed onset potential is 385, 360, 360, 360, 350 and 320 mV [vs. Ag/AgCl] for the nanofibers obtained from electrospun solutions having 0, 1, 2, 5, 7 and 9 wt% urea, respectively. Therefore, it can be claimed that increasing the nitrogen content in the introduced Ni-decorated carbon nanofibers enhances the onset potential. However, for the utilized urea solution concentrations, the maximum current density, which reflects the maximum oxidation rate of the urea molecules, was obtained with the 5 wt% urea sample as shown in Fig. 9. Fig. 9A demonstrates the impact of urea content in the original electrospun solution on the electrocatalytic activity of
Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076
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doping strategy can decrease the onset potential of urea oxidation which save the required energy for the urea electrolysis process. However, the urea content in the initial electrospun solution should be optimized.
Acknowledgement This paper was made possible by NPRP grant # [8-1344-1-246] from the Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of the authors.
references
Fig. 10 e Chronoamperogram of electroactivity of N (2%)doped NiCNFs electrode at an oxidation potential 0.4 V for urea electrooxidation in the 1 M KOH and 0.33 M urea aqueous solution at room temperature.
the prepared nanofibers toward 0.5 M urea solution. Moreover, as shown in the remaining panels in Fig. 9, the advantage of the optimum nanofibers appears clearly at the high urea concentrations. Accordingly, it can be concluded that the optimum urea content is 5 wt%. Stability of the anode material is another important parameter. Chronoamperometry analysis is widely used measurement to investigate the stability of the electrodes as it shows the change in the measured current density at a constant voltage with the elapsed time. Fig. 10 depicts the chronoamperometry analysis results for the N-doped (2 wt%) and Ni-decorated carbon nanofibers at 0.4 V (vs. Ag/AgCl). The initial sharp decrease in the current density is attributed to the fast oxidation of the urea molecules nearby the anode surface. Afterward, the current density is limited by the mass transfer process. Actually, for all formulations, one electrode was utilized to perform all the electrochemical measurements. Therefore, considering the long test time (10,000 s), and the observed very small decrease in the current density, it can be claimed that the introduced electrodes have acceptable stability. It is noteworthy mentioning that similar results were obtained with other electrodes.
Conclusion Addition of urea to the nickel acetate/poly(vinyl alcohol)/ water sol-gel does not strongly affect the nanofibrous morphology of the electrospun nanofibers and after calcination under argon atmosphere. Moreover, due to the added urea and the unusual decomposition of the nickel acetate, the calcination process leads to produce good morphology nitrogen-doped and nickel-decorated carbon nanofibers. Nitrogen doping distinctly enhances NiOOH formation on the surface of the nickel nanoparticles which reflects good electrooxidation activity of urea. Moreover, the proposed nitrogen
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[24] Barakat NA, Khalil KA, Mahmoud IH, Kanjwal MA, Sheikh FA, Kim HY. CoNi bimetallic nanofibers by electrospinning: nickel-based soft magnetic material with improved magnetic properties. J Phys Chem C 2010;114:15589e93. [25] Yousef A, Barakat NAM, Al-Deyab SS, Nirmala R, Pant B, Kim HY. Encapsulation of CdO/ZnO NPs in PU electrospun nanofibers as novel strategy for effective immobilization of the photocatalysts. Colloids Surf A Physicochem Eng Asp 2012;401:8e16. [26] Ewen RJ, Honeybourne CL. X-ray photoelectron spectroscopy of clean and gas-doped films of phthalocyanines. J Phys Condens Matter 1991;3:S303. [27] Rahim A, Abdel Hameed R, Khalil M. Nickel as a catalyst for the electro-oxidation of methanol in alkaline medium. J Power Sources 2004;134:160e9. [28] Hahn F, Beden B, Croissant M, Lamy C. In situ UV visible reflectance spectroscopic investigation of the nickel electrode-alkaline solution interface. Electrochim Acta 1986;31:335e42. M. Voltammetry and anodic stability of a hydrous [29] Vukovic oxide film on a nickel electrode in alkaline solution. J Appl Electrochem 1994;24:878e82. [30] Fleischmann M, Korinek K, Pletcher D. The oxidation of organic compounds at a nickel anode in alkaline solution. J Electroanal Chem Interfacial Electrochem 1971;31:39e49. [31] Enea O. Molecular structure effects in electrocatalysiseII. Oxidation of d-glucose and of linear polyols on Ni electrodes. Electrochim Acta 1990;35:375e8. [32] Bard A, Faulkner L. Electrochemical methods, ch. 12. New York: Wiley; 2001.
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Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation Nasser A.M. Barakat a,b,c,*, Mohamed A. Yassin a,b, Ahmed S. Yasin a,b, Saeed Al-Meer d a
Organic Materials and Fiber Engineering Department, Chonbuk National University, Jeonju 561-756, Republic of Korea b Bionanosystem Engineering Department, Chonbuk National University, Jeonju 561-756, Republic of Korea c Chemical Engineering Department, Faculty of Engineering, Minia University, El-Minia, Egypt d Central Laboratory Unit, Qatar University, P. O. Box: 2713, Doha, Qatar
article info
abstract
Article history:
Overall, boosting the formation of the active NiOOH layer distinctly improves the electro
Received 22 March 2017
catalytic activity of nickel-based materials. Moreover, due to the adsorption capacity, the
Received in revised form
carbonaceous supports improve the electrocatalytic activities of several function materials
6 July 2017
including nickel. In this manuscript, nitrogen doping is introduced as an effective strategy
Accepted 7 July 2017
to enhance formation of the NiOOH active layer on the surface of nickel nanoparticles
Available online xxx
decorating carbon nanofibers. Typically, addition of urea to the nickel acetate/poly(vinyl alcohol)/water sol-gel leads to obtain nitrogen-doped and nickel-decorated carbon nano-
Keywords:
fibers after electrospinning the prepared solution and calcination the nanofiber mats under
N-doped nanofibers
Ar at 750 C. The invoked characterizations including XRD, TEM, SEM and XPS affirmed
Urea electrooxidation
formation of smooth nitrogen-doped carbon nanofibers decorated by crystalline nickel
Nickel-decorated carbon nanofibers
nanoparticles. Nitrogen doping strongly enhanced the formation of NiOOH active layer on
Electrospinning
the surface of the metallic nanoparticles which distinctly improved the electrocatalytic activity toward urea oxidation. However, the electrochemical measurements indicated that the content of the urea in the initial electrospun solution should be optimized as the results showed that the best nickel surface activation as well as the maximum observed current density can be achieved when the urea content is kept at 5 wt%. Typically, the observed current densities using 1.0 M urea (in 1 M KOH) were 3.15, 29.7, 13.75, 47.55, 14.13 and 9.85 mA/cm2 for the nanofibers obtained from electrospun solutions having 0, 1, 2, 5, 7 and 9 wt% urea, respectively. Moreover, the proposed nitrogen doping strategy leads to perform a distinguish decrease in the onset potential of the urea electrooxidation which reduces the required energy for the electrolysis process. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Organic Materials and Fiber Engineering Department, Chonbuk National University, Jeonju 561-756, Republic of Korea. E-mail address: [email protected] (N.A.M. Barakat). http://dx.doi.org/10.1016/j.ijhydene.2017.07.076 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076
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Introduction Urea-contaminated water creates a serious problem as it is produced in large amounts from the fertilizer industries. It is known that production of 1 ton of the urea fertilizer leads to discharge 0.5 ton of the urea-polluted water. Moreover, human and animal urines can be considered continuous and big sources of the urea-containing wastewater. Urea is not directly toxic, however its hydrolysis into ammonia results in toxicity of both marine and animal life. The ammonium can be oxidized by two groups of bacteria (Nitrobacter and Nitrosomonas), in a process called nitrification, to NO3 with an unstable intermediate product (NO2) [1]. Denitrification takes place under oxygen-shortage media such as wetlands where NO3 is reduced to various gaseous products. Moreover, urea contamination can trigger algae of the oceans to produce domoic acid; a deadly toxin [2]. In contrast to other water pollutants, urea can be exploited as a rich source of hydrogen. Typically, hydrogen content in urea is around 6.67 wt%. In other words, the urea-polluted wastewaters can be theorized as a green and renewable source of energy if the urea content could be decomposed to hydrogen and other environmentally safe gaseous byproducts (CO2 and N2) which also interprets a simultaneous purification. Moreover, compared with other liquid/gas carriers of the hydrogen, urea possesses a variety of distinguished properties such as non-toxic, stable, renewable and non-flammable [3e5]. Electrolysis is an effective and cheap methodology to extract the embedded hydrogen from urea. This new strategy
was proposed to convert urea to hydrogen directly through an electrochemical oxidation based on these reactions [6e9]: Anode : Cathode : Overall :
COðNH2 Þ2 þ 6OH /N2 þ 5H2 O þ CO2 þ 6e 6H2 O þ 6e/3H2 þ 6OH COðNH2 Þ2 þ H2 O/N2 þ 3H2 þ CO2
(1) (2) (3)
Paradoxically, precious metals show an excellent catalytic performance in the electrooxidation of several alcohols, but poor activity has been observed with urea [10]. On the other hand, nickel-based materials, which have lower electrocatalytic activity toward the alcohols compared to the precious metals, revealed higher performance toward urea electrooxidation [11e13]. The main reason behind the precedence of nickel-based materials over Pt toward urea electrooxidation is the associated NiOOH layer which can be generated on the surface by successive sweeping in an alkaline medium. On the other hand, the catalytic power of the precious metals comes from their surface electronic structure. In more details, in contrast to alcohols which require only water molecules to achieve a complete oxidation (e.g. methanol; CH3 OH þ H2 O ¼ CO2 þ 6Hþ þ 6e), urea needs OH group to be oxidized as shown in the aforementioned anode reaction (Eq. (1)) [14]. Therefore, NiOOH is considered a reactant in the urea oxidation reaction [14]. Accordingly, it can be claimed that enhancement of NiOOH formation can strongly improves urea electrooxidation. Likened alcohols oxidation reactions, immobilization of the functional materials on carbonaceous supports distinctly
Fig. 1 e SEM images for the (A) electrospun NiAc/PVA nanofibers, and the nanofibers obtained from calcination of electrospun solutions having (B) 0%, (C) 1%, and (D) 2% urea. Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076
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introduced catalyst has featured activity toward urea electrooxidation due to boosting NiOOH formation, however the content of urea in the original electrospun solution should be optimized.
Experimental details Materials Nickel (II) acetate tetrahydrate (NiAc, 98%, SamChun Chemicals Co Ltd., South Korea), urea (99%, SamChun Chemicals Co Ltd., South Korea), and poly(vinyl alcohol) (PVA; MW ¼ 89,000 g/mol, DC-Chemical Co., Republic of Korea) were used as received without further modifications.
Experimental work Fig. 2 e XRD pattern for the powder obtained from calcination of NiAc/PVA electrospun nanofibers under argon atmosphere at 750 C.
enhances the anodic reaction due to the adsorption capacity [15e18]. Compared to other nanostructures, the large axial ratio gives the nanofibrous morphology good advantage in improvement the electrocatalytic activity [19e21]. From various reported nanofibers making processes, electrospinning attracted the maximum attention due to its high yield, simplicity, applicability and durability [22,23]. Our previous studies indicated that presence of transition metals (especially nickel and cobalt) strongly enhances graphitization of the poly(vinyl alcohol) (PVA) and overcomes the normal full decomposition at low temperatures due to formation of low molecular weight compounds [24]. Therefore, with an improved graphitization, PVA can be exploited as a carbon-rich and cheap precursor for fabrication of the carbon nanofibers; carbon content in PVA is ~48 wt%. This study introduces nitrogen doping as a novel and efficient strategy to enhance formation of the NiOOH layer for nickel nanoparticles-incorporated carbon nanofibers. The proposed electrocatalyst was prepared by electrospinning of urea/poly(vinyl alcohol)/nickel acetate aqueous sol-gel followed by sintering under Ar atmosphere at high temperature; 750 C. The electrochemical measurements revealed that the
First, a stock of NiAc (20 wt%) aqueous solution was prepared. To study the influence of urea content, 0, 0.05, 0.1, 0.25, 0.35 and 0.45 g of urea was dissolved in 5 g of the prepared NiAc solution. Then, each urea/NiAc solution was mixed with 15 g of an aqueous PVA (10 wt%) solution. Typically, the utilized urea amounts were estimated to get final electrospun solutions having 0, 1, 2, 5, 7 and 9 wt% urea. The solutions were vigorously stirred at 50 C for 5 h. The electrospinning process was achieved in a simple set-up at 20 kV with a 15-cm distance between the needle of a syringe containing the solution and the nanofibers collector. The obtained electrospun mats were vacuously dried at 80 C overnight. The calcination process was done in a tube furnace at 750 C with holding time of 5 h under Ar atmosphere.
Characterization The chemical structure of the produced nanofibers was investigated by using Rigaku X-ray diffractometer (XRD, Rigaku, Japan) with Cu Ka (l ¼ 1.540 A) radiation over Bragg angle ranging from 10 to 100 . The nanofibrous morphology was studied by SEM (scanning-electron-microscope; JEOLJSM-5900, Japan). High resolution image was obtained with transmission electron microscope (TEM, JEOL JEM-2010, Japan) operated at 200 kV. X-ray Photoelectron Spectroscopy analysis (XPS, AXIS-NOVA, Kratos analytical Ltd., UK) was used to study the surface chemistry of the studied nanofibers. Cyclic
Fig. 3 e (A) Normal and (B) high resolution TEM images for the NieC nanofibers. Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076
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Results and discussion
Fig. 4 e XPS spectrum for the nanofibers obtained from calcination of (A) NiAc/PVA e 1 wt% urea and (B) NiAc/ PVA e 5 wt% urea electrospun nanofibers under argon atmosphere at 750 C. The inset in panel (A) displays the N1S peak with the background.
voltammetry analyses were performed in three electrodes cell controlled by VersaStat4 instrument. Generally, the urea was dissolved in 1 M KOH solution and the sweep potential range was kept to start from 0.2 to 1.0 V [vs. Ag/AgCl]. Conventional three electrodes cell was used in which platinum wire, glassy carbon and Ag/AgCl electrodes were used as counter, working and reference electrode, respectively. The current density was normalized to the working electrode active area (0.073 cm2). The working electrode was prepared by adding 0.002 g of the prepared nanofibers to a solution consisting of 5 mL Nafion solution (5 wt%) and 400 mL isopropanol. 15 mL of the prepared slurry was poured on the active area of the working electrode. Then, the electrode was dried at 80 C. The mass of the functional material on the surface of the working electrode was determined as 0.073 mg.
Electrospinning shows distinct performance in production of the polymer nanofibers either from solutions or melts. In case of inorganic nanofibers, the utilized metal precursor and polymer have strong impact on the nanofibrous morphology of the final product. Generally, metallic precursors having high polycondensation characteristic reveal good nanofibers. Accordingly, the alkoxides are the most widely used precursors. However, recently, acetates proved that the relatively simple polycondensation feature compared to alkoxides can also provide good nanofibrous morphology. Accordingly, several metals acetates could be utilized to produce good morphology inorganic nanofibers [25]. In this study, using nickel acetate with poly(vinyl alcohol) reveals good electrospun nanofibers as shown in the SEM image (Fig. 1A). Moreover, addition of urea (up to the used 9 wt%) did not affect the obtained nanofibrous morphology (data are not shown). Due to the aforementioned polycondensation tendency of the nickel acetate, after the calcination process, there is no strong influence on the nanofibrous morphology as shown in Fig. 1B which displays the nanofibers produced from calcination of electrospun nanofibers obtained from urea-free NiAc/PVA solution. Interestingly, as in the case of the electrospun nanofibers, addition of urea did not strongly affect the nanofibrous morphology after the calcination process as shown in Fig. 1C and D. However, crosslinking was observed among some nanofibers as shown in the figure. It is noteworthy mentioning that the crosslinking was also observed in the other formulations which are not presented in the figure. XRD is highly trustable analytical technique to investigate the chemical composition of the inorganic materials. As shown in Fig. 2, the obtained XRD pattern confirms formation of pure Ni and carbon. Typically, the standard diffraction peaks of nickel appear clearly at 2q of 44.5 , 51.8 , 76.4 and 92.9 corresponding to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) crystal plans, respectively (PDF# 04-0850). The broad peak at 2q of A 26.3 corresponding to an experimental d spacing of 3.37 reveals presence of graphite-like carbon (d (002), PDF#; 411487). It is noteworthy mentioning that similar XRD patterns were obtained with all formulations. Fig. 3A demonstrates normal TEM image for the prepared nanofibers (N-free). As shown, numerous crystalline nanoparticles are distributed along with the amorphous nanofiber matrix. The crystallinity of the observed nanoparticles could be determined by high resolution TEM image; Fig. 3B. The image refers to high crystallinity due to clear appearance of the parallel crystal planes. Moreover, as shown in the inset, the distance between two successive planes matches the standard value of the pristine nickel metal. On the other hand, the amorphous nanofiber matrix can be assigned to the detected graphite in the XRD pattern. Under the utilized high calcination temperature, it is expected that urea has been decomposed to nitrogen and other compounds. Presence of nitrogen in the introduced nanofibers was investigated by XPS analysis. As shown in Fig. 4A and B which display the XPS spectra of the nanofibers obtained from electrospun solutions containing 1 and 5 wt%, respectively, the observed peak at a binding energy of ~399 eV representing
Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076
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Fig. 5 e Surface activation by cyclic voltammetry process for the nanofibers obtained from electrospun solutions having (A) 0 wt%, (B) 5 wt%, (C) 7 wt% and (D) 9 wt% urea in presence of 1 M KOH at scan rate of 100 mV/s and 25 C.
the N1S electron confirms presence of nitrogen in the investigated samples [26]. The inset in Fig. 4A demonstrates the N1S peak with the background. It is known that XPS is a surface analyzing technique. In more details, XPS can detect the elemental composition within a 10e12 nm surface layer. Therefore, XPS was invoked as qualitative analysis technique and its quantitative analyses results cannot be exploited as a real representative for the nitrogen content of the whole materials. However, based on the XPS results, the nitrogen content in the nanofibers obtained from the electrospun solution having 5 wt% urea is 2.2 At%. Overall, based on the utilized physicochemical characterizations, it can be concluded that the introduced nanofibers are nitrogen-doped and nickel-decorated carbon nanofibers. As it was aforementioned, surface activation can be achieved by sweeping within a proper potential window in an alkaline medium. As examples, the cyclic voltammetric behaviors of four prepared nanofibers are shown in Fig. 5.
Polarization was started by a potential scanning at a scan rate of 100 mV/s from 1000 mV to 200 mV (vs. Ag/AgCl). Overall, for the nickel-based materials, there are two peaks are observed in the activation voltammogram; the first one appears at the negative potential is attributed to formation of nickel hydroxide [10,21,27], the first peak cannot be observed in Fig. 5 as it is beyond the used potential window: Ni þ 2OH 4NiðOHÞ2 þ 2e
(4)
The current densities of the redox peaks of this transformation are usually small from the beginning cycle and then the peaks could not be distinguished in the subsequent cycles [27e29]. The second peaks region is observed at the positive potential and related to the oxidation of Ni(OH)2 to NiOOH. The representing peaks are always strong and the reaction can be explained by this equation [29e31]: NiðOHÞ2 þ OH 4NiOOH þ H2 O þ e
(5)
Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076
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Fig. 6 e (A) 10th cycle in the surface activation voltammograms for all the prepared nanofibers in presence of 1 M KOH at scan rate of 100 mV/s and 25 C, and (B) influence of the urea content on the current density of the anodic peak.
point to small formation for the NiOOH active layer. On the other hand, in case of addition 5 wt% urea to the electrospun solution (Fig. 5B), the peak started at a high current density which is distinctly enhanced with increasing the number of cycles. Further addition of urea results in enhancing the peak current as shown in Fig. 5C and D which display the activation voltammograms of the nanofibers obtained from electrospun mats having 7 and 9 wt% urea, respectively. However, the peak maximum current density for both formulations is less than that in case of 5 wt% urea sample (Fig. 5B). Fig. 6A shows 10th cycle in the activation voltammograms for the all prepared nanofibers. As shown in Fig. 6B, the nanofibers obtained from electrospun solution having 5 wt% urea reveal the maximum anodic current density for the Ni(OH)2/NiOOH transformations. The NiOOH surface coverage (G) in mol/cm2 can be determined from this equation [32]. Fig. 7 e Influence of urea content on the surface coverage of the prepared nanofibers. Observable increase of the peak current at the cathodic reaction can be noticed with increasing the number of sweeping cycles which indicates enhancing the entry of OH into the Ni(OH)2 surface layer. Consequently, progressive formation of a thick layer from NiOOH can be performed [27]. Therefore, the observed redox peaks in Fig. 5 can be assigned to Ni(OH)2/NiOOH transformation process. However, as shown in the figure, the urea content in the original electrospun solution has a distinct influence on the peaks current density and its improvement upon increasing the cycle number. In more details, as shown in Fig. 5A which displays the voltammograms of the nanofibers obtained from urea-free electrospun solution, there is a very little increase in the peak current density with increasing the number of the CV cycle. Moreover, the corresponding current density is relatively small compared to the other formulations. These findings
IP ¼
2 2 n F yAG 4RT
(6)
where Ip is the peak current, n is the number of electrons sharing in the reaction, v is the scan rate, T is the temperature in Kelvin, R is the universal gas constant, F is the Faraday's constant, and A is the electrode area. Fig. 7 shows a histogram the NiOOH surface coverage for the prepared nanofibers. As shown in the histogram, addition of urea to the electrospun solution enhances NiOOH formation up to 7 wt%. However, more increase in the urea content (i.e. 9 wt%) has a negative impact on the surface activation. Interestingly, compared to nitrogen-free nanofibers, addition of 5 wt% urea to the electrospun solution leads to increase the amount of the formed NiOOH four times which reflected distinct improvement in the urea electrocatalytic activity as it will be discussed later. As it is known that the urea concentration can affect the electrooxidation reaction. Fig. 8 demonstrates the influence of urea solution concentration (in 1 M KOH) in case of nanofibers prepared from different electrospun solutions based on the
Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076
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Fig. 8 e Electrocatalytic activity at different urea concentrations for the nanofibers obtained from electrospun solutions having (A) 0%, (B) 1%, (C) 2%, (D) 5%, (E) 7% and (F) 9% urea in presence of 1 M KOH at scan rate of 50 mV/s and 25 C.
Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076
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Fig. 9 e Electrocatalytic activities for all the prepared nanofibers in presence of (A) 0.5 M, (B) 1.0 M, (C) 3.0 M and (D) and 4.0 M urea (in 1 M KOH at scan rate of 100 mV/s and 25 C).
urea content. Overall, based on the reaction stoichiometry, low urea concentration reveals high catalytic activity. This hypothesis is confirmed with most of the synthesized nanofibers as displayed in the figure. Interestingly, the results also indicated that there is a distinct influence of the content of the nitrogen in the prepared nanofibers on the urea electrooxidation process. Moreover, it can be observed that the general trend in the obtained results resembles to the case of the activation process (Figs. 5 and 7). In more details, the sample owing the best activation process (5 wt% urea sample) shows the maximum electrocatalytic activity toward urea oxidation (Fig. 8D). Furthermore, nitrogen doping leads to have a clear appearance for the urea oxidation peak as shown in all nitrogen-contained nanofibers. Urea oxidation peak is very clear at the low nitrogen content nanofiber (sample obtained from the electrospun solution having 1% urea; Fig. 8B), and also at the maximum utilized urea content (9 wt%; Fig. 8F). Moreover, the urea oxidation peak shifted to negative direction after nitrogen doping for all formulations.
Theoretically, urea electrolysis reaction requires relatively high energy due to the considerable low cathode reaction potential (water reduction reaction (Eq. (2))). Accordingly, owing low onset potential is an important characteristic for the anode material as it leads to decrease the required power. In this regard, decreasing the onset potential upon nitrogen doping which is observed in Fig. 8 (marked by the arrows) is considered another important advantage. Numerically, the observed onset potential is 385, 360, 360, 360, 350 and 320 mV [vs. Ag/AgCl] for the nanofibers obtained from electrospun solutions having 0, 1, 2, 5, 7 and 9 wt% urea, respectively. Therefore, it can be claimed that increasing the nitrogen content in the introduced Ni-decorated carbon nanofibers enhances the onset potential. However, for the utilized urea solution concentrations, the maximum current density, which reflects the maximum oxidation rate of the urea molecules, was obtained with the 5 wt% urea sample as shown in Fig. 9. Fig. 9A demonstrates the impact of urea content in the original electrospun solution on the electrocatalytic activity of
Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076
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doping strategy can decrease the onset potential of urea oxidation which save the required energy for the urea electrolysis process. However, the urea content in the initial electrospun solution should be optimized.
Acknowledgement This paper was made possible by NPRP grant # [8-1344-1-246] from the Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of the authors.
references
Fig. 10 e Chronoamperogram of electroactivity of N (2%)doped NiCNFs electrode at an oxidation potential 0.4 V for urea electrooxidation in the 1 M KOH and 0.33 M urea aqueous solution at room temperature.
the prepared nanofibers toward 0.5 M urea solution. Moreover, as shown in the remaining panels in Fig. 9, the advantage of the optimum nanofibers appears clearly at the high urea concentrations. Accordingly, it can be concluded that the optimum urea content is 5 wt%. Stability of the anode material is another important parameter. Chronoamperometry analysis is widely used measurement to investigate the stability of the electrodes as it shows the change in the measured current density at a constant voltage with the elapsed time. Fig. 10 depicts the chronoamperometry analysis results for the N-doped (2 wt%) and Ni-decorated carbon nanofibers at 0.4 V (vs. Ag/AgCl). The initial sharp decrease in the current density is attributed to the fast oxidation of the urea molecules nearby the anode surface. Afterward, the current density is limited by the mass transfer process. Actually, for all formulations, one electrode was utilized to perform all the electrochemical measurements. Therefore, considering the long test time (10,000 s), and the observed very small decrease in the current density, it can be claimed that the introduced electrodes have acceptable stability. It is noteworthy mentioning that similar results were obtained with other electrodes.
Conclusion Addition of urea to the nickel acetate/poly(vinyl alcohol)/ water sol-gel does not strongly affect the nanofibrous morphology of the electrospun nanofibers and after calcination under argon atmosphere. Moreover, due to the added urea and the unusual decomposition of the nickel acetate, the calcination process leads to produce good morphology nitrogen-doped and nickel-decorated carbon nanofibers. Nitrogen doping distinctly enhances NiOOH formation on the surface of the nickel nanoparticles which reflects good electrooxidation activity of urea. Moreover, the proposed nitrogen
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Please cite this article in press as: Barakat NAM, et al., Influence of nitrogen doping on the electrocatalytic activity of Ni-incorporated carbon nanofibers toward urea oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.07.076