Scholarly article on topic 'Facile synthesis of Ni-decorated multi-layers graphene sheets as effective anode for direct urea fuel cells'

Facile synthesis of Ni-decorated multi-layers graphene sheets as effective anode for direct urea fuel cells Academic research paper on "Chemical sciences"

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{Graphene / Nickel / Nanocomposites / "Urea electrooxidation" / "Fuel cell"}

Abstract of research paper on Chemical sciences, author of scientific article — Ahmed Yousef, Mohamed H. El-Newehy, Salem S. Al-Deyab, Nasser A.M. Barakat

Abstract A large amount of urea-containing wastewater is produced as a by-product in the fertilizer industry, requiring costly and complicated treatment strategies. Considering that urea can be exploited as fuel, this wastewater can be treated and simultaneously exploited as a renewable energy source in a direct urea fuel cell. In this study, multi-layers graphene/nickel nanocomposites were prepared by a one-step green method for use as an anode in the direct urea fuel cell. Typically, commercial sugar was mixed with nickel(II) acetate tetrahydrate in distilled water and then calcined at 800°C for 1h. Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS) were employed to characterize the final product. The results confirmed the formation of multi-layers graphene sheets decorated by nickel nanoparticles. To investigate the influence of metal nanoparticles content, samples were prepared using different amounts of the metal precursor; nickel acetate content was changed from 0 to 5wt.%. Investigation of the electrochemical characterizations indicated that the sample prepared using the original solution with 3wt.% nickel acetate had the best current density, 81.65mA/cm2 in a 0.33M urea solution (in 1M KOH) at an applied voltage 0.9V vs Ag/AgCl. In a passive direct urea fuel cell based on the optimal composition, the observed maximum power density was 4.06×10−3 mW/cm2 with an open circuit voltage of 0.197V at room temperature in an actual electric circuit. Overall, this study introduces a cheap and beneficial methodology to prepare effective anode materials for direct urea fuel cells.

Academic research paper on topic "Facile synthesis of Ni-decorated multi-layers graphene sheets as effective anode for direct urea fuel cells"

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Original article

Facile synthesis of Ni-decorated multi-layers graphene sheet2s as effective anode for direct urea fuel cells

Ahmed Yousef, Mohamed H. El-Newehy, Salem S. Al-Deyab, Nasser A.M. Barakat

PII: DOI:

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S1878-5352(16)30265-9 http://dx.doi.org/10.1016/j.arabjc.2016.12.021 ARABJC 2033

Arabian Journal of Chemistry

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Arabian Journal of Chemistry

Received Date: Revised Date: Accepted Date:

20 October 2016 26 December 2016 28 December 2016

Please cite this article as: A. Yousef, M.H. El-Newehy, S.S. Al-Deyab, N.A.M. Barakat, Facile synthesis of Ni-decorated multi-layers graphene sheet2s as effective anode for direct urea fuel cells, Arabian Journal of Chemistry (2017), doi: http://dx.doi.org/10.1016Zj.arabjc.2016.12.021

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Facile synthesis of Ni-decorated multi-layers graphene sheets

îeets

as eftectlve anode for dlrect ure* fuel cells

Ahmed Yousef1, Mohamed H. El-Newehy2' 3*, Salem S. Al-Deyab2, Nasser A. M.

Barakat1' 4' *

1Bionanosystem Engineering Department, College of Engineering, Chonbuk National University, Jeonju,

South Korea.

2Petrochemical Research Chair, Department of Chemistry, College of Science, King Saud

University, Riyadh 11451, Saudi Arabia 3Department of Chemistry, Faculty of Science, Tanta University, Tanta 31527, Egypt 4Chemical Engineering Department, Faculty of Engineering, Minia University, El Minia, Egypt.

Corresponding authors: Nasser A. M. Barakat (nasser@jbnu.ac.kr) Mohamed H. El-Newehy (melnewehy@ksu. edu. sa)

Address: Petrochemical Research Chair, Department of Chemistry, College of Science, King Saud

University, Riyadh 11451, Saudi Arabia.

Facile synthesis of Ni-decorated multi-layers graphene sheets as effective anode for direct urea fuel cells

Ahmed Yousef1, Mohamed H. El-Newehy2' 3*, Salem S. Al-Deyab2, Nasser A. M.

Barakat

1, 4, *

Bionanosystem Engineering Department, College of Engineering, Chonbuk National University, Jeonju,

South Korea.

2Petrochemical Research Chair, Department of Chemistry, College of Science, King Saud

University, Riyadh 11451, Saudi Arabia 3Department of Chemistry, Faculty of Science, Tanta University, Tanta 31527, Egypt 4Chemical Engineering Department, Faculty of Engineering, Minia University, El Minia, Egypt.

ser@jbnu.ac.kr)

Corresponding authors: Nasser A. M. Barakat (nasser@jbnu.ac.kr) Mohamed H. El-Newehy (melnewehy@ksu. edu. sa)

Address: Petrochemical Research Chair, Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia.

Abstract

A large amount of urea-containing wastewater is produced as a by-product in the fertilizers industry, requiring costly and complicated treatment strategies. Considering that urea can be exploited as fuel, this wastewater can be treated and simultaneously exploited as a renewable energy source in a direct urea fuel cel. In this study, multi-layers graphene/nickel nanocomposites were prepared by a one-step green method for use as an anode in the direct urea fuel cell. Typically, commercial sugar was mixed with nickel(II) acetate tetrahydrate in distilled water and then calcined at 800 oC for 1 h. Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS) were employed to characterize the final product. The results confirmed the formation of multi-layers graphene sheets decorated by nickel nanoparticles. To investigate the influence of metal nanoparticles content, samples were prepared using different amounts of the metal precursor; nickel acetate content was changed from 0 to 5 wt%. Investigation of the electrochemical characterizations indicated that the sample prepared using the original solution with 3 wt% nickel acetate had the best current density; 81.65 mA/cm2 in a 0.33 M urea solution (in 1 M KOH) at an applied voltage 0.9 V vs Ag/AgCl. In a passive direct urea fuel cell based on the optimal composition, the observed maximum power density was 4.06^10 mW/cm with an open circuit voltage of 0.197 V at room temperature in an actual electric circuit. Overall, this study introduces a cheap and beneficial methodology to prepare effective anode materials for direct urea fuel cells.

Keywords: Graphene; Nickel; Nanocomposites; Urea electrooxidation; Fuel cell.

1. Introduction

Owing to the depletion of fossil fuels, researchers turn to utilizing different strategies to develop new energy devices. Exploiting wastewaters, such as urea-contaminated water, for power generation is highly recommended as it provides an additional advantage for the environment. Besides animal and human urine, industrial plants produce large amounts of urea-polluted wastewaters. For instance, in the urea synthesis process from ammonia and carbon dioxide, for each mole of urea synthesized, 1 mol of water is formed in addition to the water used in the feed (~ 0.7 mol/mol of urea). The produced wastewater contains ~ 0.3-1.5 wt % urea. In this wastewater, decomposition of urea to ammonia, nitrogen oxides, and nitric acid can contribute to pollution by leading to acid rain. Therefore, this wastewater should be treated before discharging which requires costly techniques. Meanwhile, urea is a promising fuel as it can be electrolyzed to produce hydrogen or used directly in fuel cells to generate electricity. The energy generated from urea is higher than that obtained from liquid or compressed hydrogen, where the theoretical efficiency of the direct urea fuel cell (DUFC), which is 102.9% at room temperature, is higher than that of hydrogen fuel cell (83% under similar conditions). The operating mechanism of DUFC can be represented by the following reactions (Lan et al. 2010b; Lan and Tao 2011; Xu et al. 2014; Barakat et al. 2016c):

Anode: C0(NH2)2 + 60H~ ^ N2 + SH20 + C02 + 6e~ (1)

E° = -0.746V vs.SHE

Cathode: 02 + 2H20 +4e~ ^ 40H~ (2)

E° = +0.401 V vs. SHE

Overall reaction: C0(NH2)2 + 302 ^ 2N2 + 2C02 + 4H20 (3)

^o = 1.147 y ps. SHE

The therotical open circuit voltage (OCV) of urea the fuel cell is 1.147 V at room temperature which is slightly lower than that of hydrogen fuel cell (1.23 V at room temperature).

Accordingly, a fuel cell stack can effectively treat urea-containing industrial wastewaters and simultaneously lead to production of considerable electrical energy. However, designing an effective electrocatalyst for the urea oxidation reaction to be exploited as anode is not an easy task. In previous studies, several kinds of materials were used as electrocatalysts for urea electro-oxidation including noble-metal based catalysts such as Ru-TiO2 (Wright et al. 1986), Ti-Pt (Simka et al. 2009), Ti-(Pt-Ir) (Simka et al. 2007), and non-noble-metal ones such as Ni (Boggs et al. 2009), boron-doped thin-film diamond and SnO2-Sb2O5 (Cataldo Hernández et al. 2014).

Nickel is an efficient catalyst for urea electrooxidation as it shows high current densities at comparatively lower overpotentials than other metals. Recently, Ni-containing electrocatalysts have witnessed rapid development; metallic Ni (Boggs et al. 2009; Lan and Tao 2011; Vedharathinam and Botte 2012), nickel nanotubes (Ji et al. 2013), nickel nanowires (Yan et al. 2014; Guo et al. 2015), nickel hydroxide (Wang et al. 2011; Wang et al. 2012; Ji et al. 2013; Vedharathinam and Botte 2013; Wu et al. 2014a), Ni-Co bimetallic hydroxide (Yan et al. 2012b; Xu et al. 2014), nickel oxide (Wu et al. 2014b), graphene oxide-nickel nanocomposites (Wang et al. 2013), Ni-graphene (Barakat et al. 2016a) (Wang et al. 2013), ionic liquid-Ni(II)-graphite composites (Chen et al. 2015), NiMoO4.xH2O nanosheets (Liang et al. 2015), porous nickel@carbon sponge (Ye et al. 2015), Ni&Mn nanoparticles (Barakat et al. 2016b), CoNi film (Vilana et al. 2016), etc.

Two-dimensional (2D) crystalline materials have a number of unique properties that make them interesting for both fundamental studies and future applications. The first material in this class is graphene; a single atomic layer of carbon. Graphene has a number of remarkable mechanical, thermal and electrical properties. Besides its excellent thermal and electrical

conductivities, graphene has a large specific surface area and excellent chemical stability which are highly preferable characteristics for support materials in composite electrocatalysts (Allen et al. 2010; Li and Kaner 2008; Rao et al. 2009). In this context, graphene-based nanocomposites catalysts are expected to improve the performance of the direct urea fuel cell electrode.

Several methods have been introduced for graphene synthesis including mechanical

exfoliation (Avouris and Dimitrakopoulos 2012; Novoselov et al. 2004), chemical vapor deposition (CVD) (Hagstrom et al. 1965), and chemical reduction of graphene oxide (Barakat and Motlak 2014a; Barakat et al. 2014; Barakat et al. 2015). The mentioned processes are the widest used ones; however they suffer from high cost, low yield, and long time-consuming procedures. Moreover, most of the introduced procedures for the synthesis of metal nanoparticles-decorated graphene nanocomposites consist of multiple-steps and use expensive precursors. From the instrumentation point of view, the chemical routes are the cheapest strategy; however, these procedures require several chemicals during the preparation steps which is disadvantageous. Industrially, utilizing commercial and abundant precursors is desirable from an economic point of view. Recently, sugar was introduced as a promising precursor for graphene with good industrial applications(Gupta et al. 2012a; Zhu et al. 2010). Accordingly, graphene has been synthesized from low-value or negatively valued raw carbon-containing materials (e.g. cookies, chocolate, grass, plastics, roaches, and dog feces) (Ruan et al. 2011). Moreover, Akhavan et al (Akhavan et al. 2014) have introduced the preparation of graphene from various

natural and industrial carbonaceous wastes such as vegetation wastes (wood, leaf, bagasse, and fruit wastes), animal wastes (bone and cow dung), a semi-industrial waste (newspaper), and industrial waste (soot powders produced in exhaust of diesel vehicles).

In this work we prepared graphene/nickel nanocomposites from inexpensive materials by a one-step method to be exploited as anode material for direct urea fuel cells. The graphene

sheets were prepared by calcination of commercial sugar at 800 oC. Similarly, the graphene/nickel nanocomposites (Gr/Ni) were prepared at different concentrations of nickel acetate; 1, 2, 3, 4 and 5 wt%. The sample prepared from 3 wt% metallic precursor exhibited higher catalytic activity than those of the other concentrations and high stability as well. A direct urea fuel cell was fabricated using Gr/Ni at 3 wt% as the anode electrode, platinum/carbon (20% of platinum) as the cathode, and an anion exchange membrane.

xchange membra

2. Experimental 2.1. Preparation of electrocatalys

A one-step synthesis method was used to prepare the electrocatalysts. Typically, 1 g of commercial sugar obtained from the local market and nickel(II) acetate tetrahydrate (Ni(CH3COO) 2.4H2O, 98%, Alfa Aesar) were used as precursors for graphene and nickel, respectively. Sugar and metallic precursor weights were estimated so as to have final solutions

containing 0, 1, 2, 3, 4 and 5 wt% nickel acetate. Later, the solid mixtures were dissolved in 20 istilled water. Then, the solutions were heated from room temperature to 800oC at a heating rate of 3 oC min-1 under an argon atmosphere with a holding time of 1 h. The obtained products were used as it is without any further treatments.

2.2. Physical characterization

Raman spectra were collected on a spectrometer (JY H800UV) equipped with an optical microscope at room temperature. For excitation, the 488 nm line from an Ar+ ion laser (Spectra Physics) was focused, with an analyzing spot of about 1 mm, on the sample under the microscope. X-ray diffraction (XRD) analysis was conducted on a Rigaku X-ray diffractometer

(XRD, Rigaku, Japan) using Cu-Ka radiation (X=0.154056 nm). The scanning electron microscopy (SEM) images were recorded on a JEOL JSM-5900 electron microscope, Japan. Transmission electron microscopy (TEM) images were taken on a JEOL JEM-2010 electron microscope, Japan, operated at 200 kV equipped with energy dispersive spectroscopy (EDX).

2.3. Electrochemical studies

Cyclic voltammetry (CV) and chronoamperometry (CA) analyses for urea electro-oxidation were controlled by a VersaStat4 potentiostat device. A typical three electrode electrochemical cell was utilized in which the graphene/nickel samples, platinum wire, and saturated Ag/AgCl electrode (0.1981 V vs. SHE) served as working, counter, and reference electrodes, respectively. Preparation of the working electrode was carried out by mixing 2 mg of the functional material, 20 |iL Nafion solution (5 wt%) and 400 |iL isopropanol. The solution was sonicated for 30 min at room temperature. Twenty five microliters from the prepared solution was cast on the active area of a glassy carbon electrode and the electrode was dried at 80oC for 30 min.

2.4. Fuel cell fabrication and analysis

Preparation of the fuel cell electrodes was executed by mixing 6 mg of Gr/Ni with 800 |iL isopropanol and 40 |iL Nafion (5 wt%) in an ultrasonic water bath for 30 minutes to obtain catalyst ink. Then, the prepared solution was loaded on a carbon cloth sheet (2.5*2.5 cm, Electro Chem. Inc., USA). After that, the coated electrode was dried in an air oven at 80 oC for 30

minutes. Similarly, the cathode was prepared by loading a suspension containing Pt/C (20 wt% Pt) nanoparticles (3 mg Pt/C in 800 |iL isopropanol and 40 |iL Nafion) on a carbon cloth sheet (2.5^2.5 cm); the loaded carbon cloth was dried at 80 oC for 30 minutes prior to serving as cathode.

Commonly, KOH solution is used as the electrolyte in conventional alkaline fuel cells, but CO2 is one of the urea electrooxidation products and the reaction between CO2 and KOH is a typical problem (Varcoe et al. 2007). Consequently, an anion exchange polymer membrane (AEM, AMI-7001, AMFOR INC.) that is a compatible with CO2 (Unlu et al. 2009) was used as electrolyte. The anion exchange membrane was immersed in 1 M KOH solution and heated at 50 oC for 2 h, and then left in the solution for 10 h as a pretreatment procedure. Gold-coated

stainless steel plates with incisions as flow channels were used as current collectors at the stainless steel plates with incisions as flow channels were used as current collectors at the

cathode and anode. Aqueous solution of 0.33 M urea in 1M KOH was fed into a chamber as a fuel at the anode (passive cell). At the cathode, the oxygen in air atmosphere was used as

Fig. 1 shows a domestic simple circuit (self-made) which was used to measure the fuel cell performance and the current-voltage characterization. This circuit reveals the fuel cell as part of an electric circuit which views I-V performance data for a fuel cell in a more useful form (Benziger et al. 2006). According to this circuit, the fuel cell serves as power source for the

circuit so it is convenient to consider it as a battery. The open circuit voltage (OCV) can expressed as the cell emf value when the internal resistance (Rin) is considered zero, or the cell voltage that can be measured in the absence of current. The cell voltage is a linear function of current, and can be described by the following equation:

Vcell = OCV - IRin = IRl (4)

electric be expr

where RL is the external load. Changing the current through the external load requires changing the external resistance. The power delivered (P) to the external load is considered the useful power which is a function of the external load resistance. Then practically, the power output (P) of a fuel cell is controlled by changing the load impedance, the power is given by:

P = Ween (5)

The I-V data is plotted according to eq.4, while the polarization curve was obtained by variation of the external resistance (Benziger et al. 2006; LOGAN et

3. Results and Discussions

Overall, one of the simplest methods to get carbon is the dehydration of sugar as an abundant precursor, where C12H22O11 is converted completely to carbon element and water molecules by heating, as shown in the following reaction. However, the presence of water in the proposed methodology can enhance the obtained carbon structure (Gupta et al. 2012b):

C12H22011 ^12C + HH20 (6)

In order to detect the structure of the produced carbon, Raman spectroscopy was invoked as an effective technique based on the location and intensity of the relative peaks. Fig. 2A shows the Raman spectra of the product obtained from Ni-containing composite (3 wt% sample) and Ni-free samples. Typical result spectra contain two peaks at 1343 and 1582 cm-1 corresponding to the well-defined D and G bands, respectively. Obviously, the D band has a slightly higher intensity than the G band which indicates the reduction in the size of the in-plane sp2 domains and confirms the formation of the graphene structure as well. Furthermore, a band located at 2196 cm-1, which is attributed to the nickel nanoparticles intercalated into graphene can be observed (Stankovich et al. 2007; Guo et al. 2009). Fig. 2B displays the Raman spectra of the

different samples. As shown, increasing the nickel content influences the intensity of the D and G peaks. Overall, Raman results indicated that the obtained graphene is multi-layered (possibly more than 10 layers). Besides these two main bands, the 2D band appearing at ~ 2680 cm-1 can provide useful information about the structure of the synthesized graphene (Akhavan 2015).

Typically, the 2D/G ratio is inversely proportional to the number of graphene layers. For instance, 2D/G > 1.6 reveals single-layer graphene, while small ratio indicates multilayer graphene (more than 5) (Kim et al. 2009; Calizo et al. 2007). Based on the obtained results, the 2D/G ratio of the nickel-free graphene was 0.46 and it decreased to ~ 0.36 after addition of nickel in all formulations. This finding indicates that the presence of nickel led to increase in the number of graphene layers.

To verify the Gr/Ni product crystal structures, crystallinity, and crystal sizes, the obtained samples were examined by X-ray diffraction analysis. Fig.3 revealed that the cubic crystal system of nickel was consistent with the standard crystallographic spectrum of nickel (Hull., Phys. Rev. 1921, Ref. Code: 01-1258) (Hull 1921). Comparing the d-spacing of the valuable X-ray card with those obtained experimentally, the obtained peaks (20) at 44.43o, 51.76o and 76.31o are in agreement with the (111), (200) and (220) planes, respectively of the standard. In addition, no states of oxides, hydroxides, or secondary phases appeared. Comparison between charts of Gr/Ni for different concentrations of Ni yielded that the peaks became higher and sharper with increasing nickel concentration, which indicates increasing of the nickel nanoparticles crystallinity. This result reveals increase in the crystal size of the nickel nanoparticles because the crystal size and the width of the peak are inversely proportional (Drits et al. 1997).

The crystallite size (D) of the obtained Ni NPs in Gr/Ni at all concentrations were calculated using the Debye-Scherer's formula (eq.7) from the full width at half-maximum (FWHM) (r) of the nickel's peaks (Scherrer 1918; Burton et al. 2009).

D cos(0) V '

where À is the wavelength of Cu-Ka radiation and 0 is the position of the peak. The average crystal size values of the nickel particles in Gr/Ni samples at 1, 2 and 3 wt% are very close; 20.38, 21.48 and 23.82 nm, respectively. However, they are smaller than the crystal size of the Gr/Ni samples at 4 and 5 wt% which are 41.32 and 43.74 nm, respectively. The diffraction peak at 26.1o corresponds to the (002) hexagonal graphite phase (Hanawalt. et al., Anal. Chem., 1938, Ref. Code: 01-0640) (Hanawalt et al. 1938). This broad peak is suggestive of a loss of long range order in the stacked layers of graphene. Formation of pure nickel rather than the expected metal oxide form can be attributed to formation of reducing gases during the abnormal decomposition of nickel acetate in the inert atmosphere. Actually, in inert atmosphere, the acetate abnormally decomposes producing strongly reducing gases (namely, CO and H2) which results in complete reduction of the salt leading to form the pure metal rather than the metal oxides (De Jesus et al. 2005; Barakat et al. 2010; Barakat et al. 2008; Barakat et al. 2009). Briefly, the formation of pure nickel was explained by the following reactions:

Ni(CH3COO)2-4H2O -► 0.86Ni(CH3C00)20.14Ni(OH)2 + 0.28CH3COOH + 3.72H2O (8)

0.86 Ni(CH3COO)2- 0.14Ni(OH)2 -► NiCOs + NiO + CH3COCH3 + H2O (9)

NiCO3 -► NiO + CO2 (10)

NiO + CO -► Ni + CO2 (11)

As the proposed synthesis process is based on using aqueous solution, the observed graphitization yield was around 17 wt%. Compared to other materials, the obtained graphitization yield is considered a low value. However, it is acceptable with respect to the utilized precursor; the carbon content in sucrose is around 26.67 wt%. On the other hand,

considering that the formed intermediates (NiO and NiCO3) have high melting points, it is safe to claim that no losses in the nickel metal occured during the decomposition of the metal salt. In other words, based on the meting point of NiO and Ni (1955 and 1455 oC, respectively) and the strong reducing power of the formed gases (hydrogen and carbon monoxide), it can be confidently claimed that all the nickel content in the utilized precursor was obtained in the final product. Accordingly, the nickel content in the final products was calculated to be 1.41, 2.79, 4.12, 5.42, and 6.69 wt% for the samples synthesized from initial solutions having 1, 2, 3, 4, and 5 wt% nickel acetate, respectively. It is noteworthy mentioning that these calculations almost match the EDX analysis results (data are not shown).

To confirm the composition and investigate the surface morphology of the obtained Gr/Ni samples, scanning electron microscope (SEM) analysis was invoked. As shown in Fig.4a, the sheets of graphene at 0 wt% consisted of thin and wrinkled sheets linked together. Fig.4b, 4c, 4d, 4e and 4f show the formation of nickel nanoparticles on the graphene sheets.

TEM results are presented in Fig. 5. Fig. 5A and 5B show the spherical shape of the inoparticles for Gr/Ni 3 wt% concentration of nickel acetate. The average particle size of nickel nanoparticles was estimated at 27.43 nm which is close to the XRD results. Furthermore, the elemental mapping images (Fig.5C, 5D and 5E) reveal that no impurities states appear in the Gr/Ni at 3 wt% sample. The proposed preparation process is considered a one-pot procedure for synthesis of metal nanoparticles-decorated graphene. Therefore, besides the good

nickel nano the nicl

application, the introduced procedure saves time and chemicals to prepare such functional materials compared to conventional multi-step routes (Barakat and Motlak 2014b).

For comparison, Fig. 6 exhibits the typical cyclic voltammetry (CV) of the Gr/Ni 0, 1, 2, 3, 4 and 5 wt% and pure Ni (Ni nanopowder <100 nm, 99.9%, Sigma-Aldrich) electrodes measured in 1 M KOH solution at a scan rate of 50 mV.s-1. Rreduction and oxidation current

sible conv

peaks were observed between 0.65 to 0.35 V, which corresponds to the reversible conversion of Ni2+ to Ni3+ according to the following reaction (Vedharathinam and Botte 2012):

d Botte

Ni(OH)m + OH' ^ NiOOH{s) + H20{l) + e' (12)

As observed from the inset figure (Fig. 6), the pure graphene (Gr/Ni at 0 wt%) does not show any reduction or oxidation current peaks, which is due to the absence of nickel in the electrode.

The oxidation current density of Gr/Ni at 1, 2, 3, 4 and 5 wt% are higher than pure Ni as shown in Fig.6. This finding can be attributed to the influence of the graphene support which increases the active surface area of the catalyst. Subsequently, it reveals more active sites and increases oxidation current. Moreover, the known high adsorption capacity can have a very efficient role. The electroactive surface areas (ESA) of the samples were estimated by the following equation (Xia et al. 2009; Lee et al. 2007; Li et al. 2011; Wang et al. 2012):

ESA = — (13)

where; q is the charge associated with the formation of a monolayer of Ni(OH)2 (=257 |iC cm-2) (Machado and Avaca 1994; Brown and Sotiropoulos 2000; Hahn et al. 1986), m is the loading amount of the catalysts, and Q is the charge required to reduce NiOOH to Ni(OH)2 (calculated from the cyclic voltammogram). The electroactive surface areas of Gr/Ni at 1, 2, 3, 4 and 5 wt%

are larger than pure Ni as shown in Table 1. This elucidates that the increase of the oxidation current density of the Gr/Ni samples is more greater the pure Ni oxidation current density.

Fig. 7 displays the voltammograms of the samples in the presence of 0.33 M urea (in 1 M KOH) at a scan rate of 50 mV/s. Compared to Fig. 6, a significant increase in the current density was obtained after urea addition for pure Ni and all the prepared formulations. As a result of the increase of the ac,ive f area of*. ^ Ca,alyStS by graphene ^ corren, de„Si,y of Gr/Ni samples are higher than the current density of pure Ni. Therefore, the graphene sheets enhance the electron transfer for urea oxidation. However, the Gr/Ni electrode obtained from the 3 wt% sample affords the highest current density (81.65 mAcm-2 at an applied voltage of 0.9 V vs. Ag/AgCl), as shown in the inset figure. This finding can be explained in two parts. First, compared to the low nickel contents (1 and 2 wt%), the best sample possesses higher functional material (Ni); this hypothesis is supported by the linear increase in the current density at low Ni content (up to 3 wt%). Second, in case of the higher contents (4 and 5 wt%), according to the XRD results, the particle size of nickel in these samples is larger than that in Gr/Ni at 3 wt%. Thus, it is reasonable that the active surface area of nickel particles in Gr/Ni at 3 wt% is higher than that of Gr/Ni at 4 and 5 wt%, and consequently the current density of Gr/Ni at 3 wt% is higher than that of Gr/Ni at 4 and 5 wt.%. The onset potential for urea electrooxidation on the Gr/Ni electrodes is approximately the same for all samples, which is around 0.38 V.

e influence of urea concentration on the catalytic performance of the Gr/Ni at 3 wt.% electrode solution was investigated (Fig. 8). According to (eq.1), the suitable ratio of [OH-]/[CO(NH2)2] is around 8, and it is appropriate for 0.125 M urea to react with 1 M KOH (Guo et al. 2015). Therefore, as the concentration of 0.33M urea is very close to the ideal ratio, the

current density obtained with the best electrode (Ni content 3 wt%) is higher than other concentrations of urea.

It is known that transition metals possess good stability in alkaline medium than acidic. To affirm this hypothesis, the stability of the Gr/Ni at 3 wt% electrode was assayed by chronoamperometry (see Fig. 9). Apparently, the electrode maintains its activity toward urea electrooxidation in 0.33 M urea / 1 M KOH solution for at least 10,000 seconds at an applied cell voltage of 0.5 V. Therefore, it can be claimed that the Gr/Ni at 3 wt% electrode has a good stability for applications.

As aforementioned, the main target of this study is building a direct urea fuel cell to generate power and simultaneously oxidize urea to environmentally safe products. As concluded from Fig. 8 and also based on the urea oxidation reaction, low urea concentration solutions are more favorable. Fig. 10 demonstrates the performance of the assembled direct urea fuel cell using the best sample as anode with 0.33 M urea solution as fuel and natural oxygen in air atmosphere as the electron acceptor. The cell was operated at room temperature to mimic industrial conditions. As shown, the cell achieved an appreciable maximum power density (Pmax) of 4.06x10"3 mW cm-2.

Actually, DUFCs show low power than other fuel cells because the internal resistance of the fuel cell is the parameter that most affects. Therefore, the internal resistance of the cell has been calculated. Equation (4) can be reduced to equation (14) to estimate the internal resistance (Rin) and the open circuit voltage (OCV) by plotting the relation between (1/RL) and (1/Vcell) as shown in Fig. 11:

1 OCV 1 1

— = -.------(14)

Rl Rin Vcen Rin

According to the eq.14 and Fig. 11, the open circuit voltage (OCV) is 197.13 mV and the internal resistance (Rin) of the DUFC is 775.19 Q, which is a very high internal resistance for a power source in an electric circuit. Actually, this high resistance can be attributed to the utilized membrane. Subsequently, most of the power was exhausted to overcome the internal resistance; hence the produced power was reduced.

Although, the produced power looks smaller than in direct alcohol fuel cells, it is believed that this is a good finding as power could be generated simultaneously with wastewater treatment. Moreover, the results emphasize the distinct role of graphene as the utilized metal amount is very small with a considerable output of power. Therefore, the interactive contribution of graphene and nickel in addition to the large active surface area of graphene sheets can

active s

promote the performance of the DUFC even at low concentrations of nickel. It is noteworthy that pure nickel acetate powder has been sintered at the same calcination temperature. The obtained power revealed similar CV results to those obtained from purchased nickel powder (Fig. 6). However, in the fuel cell, no power was obtained as a negative OCV was obtained.

There was a big difference between the current density obtained from the CV measurements and from the cell. The big decrease in the current density can be attributed to internal cell resistance. The internal resistance (Rint) can be determined from the linear polarization curve, where it equals the slope of this curve (AE/AI); ~ 2670 Q. This value is considered the average resistance of the cell; it is a very big value for the cell resistance and it can be attributed to the utilized commercial anion exchange membrane. Overall, the instant internal resistance depends on the external load and the generated current density and can be divided into three regions. The first appears at low current and high potential in the polarization curve. Within this region, the generated electrons need to overcome the back potential provided

to the cell from the external load to produce current. The second zone, called the ohmic resistance zone, has a vital role in determining the point of the maximum achievable power. The ohmic resistance is mainly related to the cell components as it represents the resistances of the electrodes, membrane, electrolyte and external connections. The third zone is called the mass

transfer resistance. Fig. 12 displays the relation between the cell resistance and the current density. As shown, at low current density (high voltage), the cell resistance was high and then decreased sharply with decreasing voltage and increasing current density. This measurement is considered a real evaluation of the cell resistance during the working stage. The results strongly encourage enhancing the electrical conductivity of the membrane which will be the next target.

Overall, the main advantage of the study is in obtaining an efficient electrocatalyst for

direct urea fuel cells from cheap and abundant precursors. Table 2 shows a comparison between the electrocatalytic performance of the introduced decorated graphene and other anodes, as can be observed, the proposed anode shows good performance.

Conclusions

ows good

Nickel nanoparticles-decorated graphene can be synthesized from commercial sugar and nickel acetate by a one-step process. Briefly, sintering of an aqueous solution composed of sugar and the metal salt led to full graphitization of sugar to form multi layered graphene sheets and complete reduction of nickel to form pristine nickel nanoparticles attached to a carbonaceous support. As an electrocatalyst for urea oxidation, to have the best activity, the metal content should be optimized 3 wt% of nickel acetate in the original solutions revealed the best performance. The proposed Ni/Gr composite can be exploited as an active anode in direct urea fuel cells for the simultaneous treatment of urea-containing industrial wastewater and power

generation. The obtained results can be attributed to the large active surface area of graphene sheets and the synergistic contribution of graphene and nickel.

Acknowledgements:

This project was funded by the National Plan for Science, Technology and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award Number (11-ENE1917-02).

References

Akhavan O, 2015, Bacteriorhodopsin as a superior substitute for hydrazine in chemical reduction

of single-layer graphene oxide sheets. Carbon 81, 158-166 Akhavan O, Bijanzad K, Mirsepah A, 2014, Synthesis of graphene from natural and industrial

carbonaceous wastes. RSC Advances 4 (39), 20441-20448 Allen MJ, Tung VC, Kaner RB, 2010, Honeycomb Carbon: A Review of Graphene. Chem Rev 110,132-145

Avouris P, Dimitrakopoulos C, 2012, Graphene: synthesis and applications. Mater Today 15 (3), 86-97

010, Hone

Barakat NA, Motlak M, 2014a, CoxNiy-decorated graphene as novel, stable and super effective

non-precious electro-catalyst for methanol oxidation. Appl Catal, B 154, 221-231 Barakat NA, Motlak M, Ghouri ZK, Yasin AS, El-Newehy MH, Al-Deyab SS, 2016a, Nickel nanoparticles-decorated graphene as highly effective and stable electrocatalyst for urea electrooxidation. J Mol Catal A: Chem 421, 83-91

Barakat NA, Motlak M, Lim BH, El-Newehy MH, Al-Deyab SS, 2014, Effective and Stable CoNi Alloy-Loaded Graphene for Ethanol Oxidation in Alkaline Medium. J Electrochem Soc 161 (12), F1194-F1201 Barakat NA, Motlak M, Nassar M, Abdelkareem MA, Mahmoud M, El-Newehy MH, Moustafa HM, 2015, From Secondary to Primary Role in Alkaline Fuel Cells: Co-Decorated Graphene as Effective Catalyst for Ethanol Oxidation. ECS Electrochem Lett 4 (1), F5-

Barakat NAM, El-Newehy MH, Yasin AS, Ghouri ZK, Al-Deyab SS, 2016b, Ni&Mn nanoparticles-decorated carbon nanofibers as effective electrocatalyst for urea oxidation. Applied Catalysis A: General 510, 180-188. doi:10.1016/j.apcata.2015.11.015 Barakat NAM, Khalil KA, Mahmoud IH, Kanjwal MA, Sheikh FA, Kim HY, 2010, CoNi Bimetallic Nanofibers by Electrospinning: Nickel-Based Soft Magnetic Material with Improved Magnetic Properties. J Phys Chem C 114 (37), 15589-15593. doi:10.1021/jp1041074

Barakat NAM, Kim B, Kim H , , Production of Smooth and Pure Nickel Metal Nanofibers by the Electrospinning Technique: Nanofibers Possess Splendid Magnetic Properties. J Phys Chem C 113 (2), 531-536. doi:10.1021/jp805692r

Barakat NAM, Kim B, Park SJ, Jo Y, Jung M-H, Kim HY, 2009, Cobalt nanofibers encapsulated

in a graphite shell by an electrospinning process. Journal of Materials Chemistry 19 (39), 7371-7378

Barakat NAM, Motlak M, 2014b, CoxNiy-decorated graphene as novel, stable and super effective non-precious electro-catalyst for methanol oxidation. Appl Catal, B 154-155 (0), 221-231. doi:http://dx.doi.org/10.1016/j.apcatb.2014.02.019

Barakat NAM, Motlak M, Ghouri ZK, Yasin AS, El-Newehy MH, Al-Deyab SS, 2016c, Nickel nanoparticles-decorated graphene as highly effective and stable electrocatalyst for urea electrooxidation. J Mol Catal A: Chem 421, doi:http://dx.doi.org/10.1016/j.molcata.2016.05.011 Benziger JB, Satterfield MB, Hogarth WHJ, Nehlsen JP, Kevrekidis IG, 2006, The power performance curve for engineering analysis of fuel cells. Journal of Power Sources 155 (2), 272-285. doi:10.1016/j.jpowsour.2005.05.049 Boggs BK, King RL, Botte GG, 2009, Urea electrolysis: direct hydrogen production from urine.

Chem Commun (Camb) (32), 4859-4861. doi:10.1039/b905974a Brown IJ, Sotiropoulos S, 2000, Preparation and characterization of microporous Ni coatings as

hydrogen evolving cathodes. Journal of Applied Electrochemistry 30, 107-111 Burton AW, Ong K, Rea T, Chan IY, 2009, On the estimation of average crystallite size of zeolites from the Scherrer equation: A critical evaluation of its application to zeolites with one-dimensional pore systems. Microporous and Mesoporous Materials 117 (1-2), 75-90. doi:10.1016/j.micromeso.2008.06.010 Calizo I, Balandin A, Bao W, Miao F, Lau C, 2007, Temperature dependence of the Raman

spectra of graphene and graphene multilayers. Nano letters 7 (9), 2645-2649 Cataldo Hernández M, Russo N, Panizza M, Spinelli P, Fino D, 2014, Electrochemical oxidation of urea in aqueous solutions using a boron-doped thin-film diamond electrode. Diamond and Related Materials 44, 109-116. doi:10.1016/j.diamond.2014.02.006 Chen J-D, Lo N-C, Huang GG, Chen P-Y, 2015, Easy-to-prepare electrochemical platform composed of ionic liquid-Ni(II)-graphite composites: laboratory study on electrochemical

oxidation of urea, alcohols, and glucose. Electrochimica Acta 182, 113-121. doi:10.1016/j.electacta.2015.09.065 De Jesus JC, González I, Quevedo A, Puerta T, 2005, Thermal decomposition of nickel acetate tetrahydrate: an integrated study by TGA, QMS and XPS techniques. J Mol Catal A: Chem 228 (1-2), 283-291. doi:http://dx.doi.org/10.1016/j.molcata.2004.09.065 Drits V, Srodon J, Eberl DD, 1997, XRD Measurement of Mean Crystallite Thickness of Illite and Illite/Smectite: Reappraisal of the Kubler Index and the Scherrer Equation. Clays and

Clay Minerals 45 (3), 461-475 Guo F, Ye K, Cheng K, Wang G, Cao D, 2015, Preparation of nickel nanowire arrays electrode

for urea electro-oxidation in alkaline medium. J Power Sources 278, 562-568 Guo HL, Wang XF, Qian QY, Wang FB, Xia XH, 2009, A green approach to the synthesis of

graphene nanosheets. ACS nano 3 (9), 2653-2659. doi:10.1021/nn900227d Gupta SS, Sreeprasad TS, Maliyekkal SM, Das SK, Pradeep T, 2012a, Graphene from sugar and

its application in water purification. ACS Appl Mater Interfaces 4 (8), 4156-4163 Gupta SS, Sreeprasad TS, Maliyekkal SM, Das SK, Pradeep T, 2012b, Graphene from sugar and its application in water purification. ACS applied materials & interfaces 4 (8), 4156-4163. doi:10.1021/am300889u Hagstrom S, Lyon H, Somorjai G, 1965, Surface structures on the clean platinum (100) surface.

Phys Rev Lett 15 (11), 491 Hahn F, Beden B, Croissant MJ, Lamy C, 1986, In situ UV visible reflectance spectroscopic investigation of the Nickel electrode-Alkaline solution interface. Electrochimica Acta 31 (3), 335-342

Hanawalt JD, Rinn HW, Frevel LK, 1938, Chemical Analysis by X-Ray Diffraction. Ind Eng

Chem Anal Ed 10 (9), 457-512 Hull AW, 1921, X-Ray Crystal Analysis of Thirteen Common Metals. Physical Review 17 (5), 571-588. doi:10.1103/PhysRev.17.571

Ji R-Y, Chan D-S, Jow J-J, Wu M-S, 2013, Formation of open-ended nickel hydroxide nanotubes on three-dimensional nickel framework for enhanced urea electrolysis. Electrochemistry Communications 29, 21-24. doi:10.1016/j.elecom.2013.01.006 Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Kim KS, Ahn J-H, Kim P, Choi J-Y, Hong BH, 2009, Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature 457 (7230), 706-710 Lan R, Tao S, 2011, Preparation of nano-sized nickel as anode catalyst for direct urea and urine fuel cells. Journal of Power Sources 196 (11), 5021-5026. doi:10.1016/j .jpowsour.2011.02.015 Lan R, Tao S, Irvine JT, 2010a, A direct urea fuel cell-power from fertiliser and waste. Energy

& Environmental Science 3 (4), 438-441 Lan R, Tao S, Irvine JTS, 2010b, A direct urea fuel cell - power from fertiliser and waste.

Energy & Environmental Science 3 (4), 438-441. doi:10.1039/b924786f Lee EP, Peng Z, Cate DM, Yang H, Campbell CT, Xia Y, 2007, Growing Pt Nanowires as a

Densely Packed Array on Metal Gauze. American Chemical Society 129, 10634-10635 Li CA, Han KN, Bui M-PN, Pham X-H, Hong MH, Irfan M, Kim YS, Seong GH, 2011, Morphology-controlled synthesis and electrocatalytic characteristics of platinum structures on micro-patterned carbon nanotube platforms. Journal of Applied Electrochemistry 41 (12), 1425-1431. doi:10.1007/s10800-011-0366-0

Li D, Kaner RB, 2008, Graphene-Based Materials. MATERIALS SCIENCE 320, 1170-1171 Liang Y, Liu Q, Asiri AM, Sun X, 2015, Enhanced electrooxidation of urea using NiMoO4-xH2O nanosheet arrays on Ni foam as anode. Electrochimica 460. doi:10.1016/j.electacta.2014.11.193 LOGAN BE, HAMELERS B, ROZENDAL R, SCHRODER U, KELLER J,

AELTERMAN P, VERSTRAETE W, RABAEY K, 2006, Microbial Fuel Cells: Methodology and Technology. ENVIRONMENTAL SCIENCE & TECHNOLOGY 40 (17), 5181-5192

Machado SAS, Avaca LA, 1994, THE HYDROGEN EVOLUTION REACTION ON NICKEL SURFACES STABILIZED BY H-ABSORPTION. Electrochimica Acta 39 (10), 13851391

Novoselov KS, Geim AK, Morozov S, Jiang D, Zhang Y, Dubonos Sa, Grigorieva I, Firsov A,

2004, Electric field effect in atomically thin carbon films. Science 306 (5696), 666-669 Rao CNR, Biswas K, Subrahmanyam KS, Govindaraj A, 2009, Graphene, the new nanocarbon.

Journal of Materials Chemistry 19 (17), 2457. doi:10.1039/b815239j Ruan G, Sun Z, Peng Z, Tour JM, 2011, Growth of graphene from food, insects, and waste. ACS nano 5 (9), 7601-7607

Scherrer P, 1918, Bestimmung der Größe und der inneren Struktur von Kolloidteilchen mittels

Röntgenstrahlen. Gottinger Nachrichten 2, 98-100 Simka W, Piotrowski J, Nawrat G, 2007, Influence of anode material on electrochemical decomposition of urea. Electrochimica Acta 52 (18), 5696-5703. doi:10.1016/j.electacta.2006.12.017

Acta 153, 4

Simka W, Piotrowski J, Robak A, Nawrat G, 2009, Electrochemical treatment of aqueous

solutions containing urea. J Appl Electrochem 39 (7), 1137-1143 Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, Wu Y, Nguyen Ruoff RS, 2007, Synthesis of graphene-based nanosheets via chemical reductio exfoliated graphite oxide. Carbon 45 (7), 1558-1565. doi:10.1016/j.carbon.2007.02.034 Unlu M, Zhou J, Kohl PA, 2009, Anion Exchange Membrane Fuel Cells: Experimental Comparison of Hydroxide and Carbonate Conductive Ions. Electrochemical and Solid-

^en ST, tion of

State Letters 12 (3), B27-B30. doi:10.1149/1.3058999 Varcoe JR, Slade RCT, Yee ELH, Poynton SD, Driscoll DJ, Apperley DC, 2007, Poly(ethylene-co-tetrafluoroethylene)-Derived Radiation-Grafted Anion-Exchange Membrane with Properties Specifically Tailored for Application in Metal-Cation-Free Alkaline Polymer Electrolyte Fuel Cells. American Chemical Society 19, 2686-2693 Vedharathinam V, Botte GG, 2012, Understanding the electro-catalytic oxidation mechanism of urea on nickel electrodes in alkaline medium. Electrochimica Acta 81, 292-300. doi:10.1016/j.electacta.2012.07.007 Vedharathinam V, Botte GG, 2013, Direct evidence of the mechanism for the electro-oxidation of urea on Ni(OH)2 catalyst in alkaline medium. Electrochimica Acta 108, 660-665. doi:10.1016/j.electacta.2013.06.137 Vilana J, Gomez E, Vallés E, 2016, Influence of the composition and crystalline phase of

electrodeposited CoNi films in the preparation of CoNi oxidized surfaces as electrodes for urea electro-oxidation. Applied Surface Science 360, 816-825. doi:10.1016/j.apsusc.2015.11.072

tion of urea

Electrochi

duced graphene

Wang D, Yan W, Botte GG, 2011, Exfoliated nickel hydroxide nanosheets for urea electrolysis. Electrochemistry Communications 13 (10), 1135-1138.

doi:10.1016/j.elecom.2011.07.016 Wang D, Yan W, Vijapur SH, Botte GG, 2012, Enhanced electrocatalytic oxidatior

based on nickel hydroxide nanoribbons. Journal of Power Sources 217, 498-502. doi:10.1016/j .jpowsour.2012.06.029 Wang D, Yan W, Vijapur SH, Botte GG, 2013, Electrochemically reduced graphene oxidenickel nanocomposites for urea electrolysis. Electrochimica Acta 89, 732-736. doi:10.1016/j. electacta.2012.11.046 Wright JC, Michaels AS, Appleby AJ, 1986, Electrooxidation of Urea at the Ruthenium

Titanium Oxide Electrode. AIChE Journal 32 (9), 1450-1458 Wu M-S, Ji R-Y, Zheng Y-R, 2014a, Nickel hydroxide electrode with a monolayer of nanocup arrays as an effective electrocatalyst for enhanced electrolysis of urea. Electrochimica Acta 144, 194-199. doi:10.1016/j.electacta.2014.08.098 Wu M-S, Lin G-W, Yang R-S, 2014b, Hydrothermal growth of vertically-aligned ordered mesoporous nickel oxide nanosheets on three-dimensional nickel framework for electrocatalytic oxidation of urea in alkaline medium. Journal of Power Sources 272, 711718. doi:10.1016/j .jpowsour.2014.09.009 Xia BY, Wang JN, Wang XX, 2009, Synthesis and Application of Pt Nanocrystals with

Controlled Crystallographic Planes. Journal of Physical Chemistry C 113, 18115-18120 Xu W, Zhang H, Li G, Wu Z, 2014, Nickel-cobalt bimetallic anode catalysts for direct urea fuel cell. Scientific reports 4, 5863-5868. doi:10.1038/srep05863

or urea

Yan W, Wang D, Botte GG, 2012a, Electrochemical decomposition of urea with Ni-based

catalysts. Appl Catal, B 127, 221-226 Yan W, Wang D, Botte GG, 2012b, Nickel and cobalt bimetallic hydroxide catalysts for urea electro-oxidation. Electrochim Acta 61, 25-30

Yan W, Wang D, Diaz LA, Botte GG, 2014, Nickel nanowires as effective catalysts for urea electro-oxidation. Electrochimica Acta 134, 266-271. doi:10.1016/j.electacta.2014.03.134 Ye K, Zhang D, Guo F, Cheng K, Wang G, Cao D, 2015, Highly porous nickel@carbon sponge as a novel type of three-dimensional anode with low cost for high catalytic performance of urea electro-oxidation in alkaline medium. Journal of Power Sources 283, 408-415. doi:10.1016/j .jpowsour.2015.02.149 Zhu C, Guo S, Fang Y, Dong S, 2010, Reducing sugar: new functional molecules for the green synthesis of graphene nanosheets. ACS nano 4 (4), 2429-2437

Fig. 1: Domestic simple circuit for the current-voltage characterization.

Raman Shift (cm )

Sample 2 wt% Sample 3 wt% Sample 4 wt%

Raman shift (cm )

Fig. 2: Raman spectra of the prepared (A) Ni-decorated and pristine graphene and (B) samples with different loading.

Gr 002

Gr/Ni at 5 wt%

Ni 220 Ni

Vr-f-Wfr' rtntmn.....nil miVii..i|.

. Gr/Ni at 4 wt%

S^HumfLrtHHrWiWiiUfciiiitlim.............A I

Gr/Ni at 3 wt%

Gr/Ni at 2 wt%

Gr/Ni at 1 wt%

Gr/Ni at 0 wt%

—i—i—|—i—|—i—|—i—|—i—|—i—|—i-20 30 40 50 60 70 80 90

Fig. 3: XRD patterns of Gr/Ni with different Ni loading (0, 1, 2, 3, 4, and 5 wt%).

3: XRD

Fig. 4: SEM images of Gr/Ni with different concentrations of Ni: (a-f) 0, 1, 2, 3, 4, and 5 wt%, respectively.

% TSS8BI

_ ^ •''.'^aar*

k'jlP'-

- j jtV

Fig. 5: TEM and EDS images of Gr/Ni with 3 wt% concentration of Nickel.

(Л d <u

706050403020100 -10-20

0.05 —I— 0.042 0.03-I

Gr/Ni 0 wt% Gr/Ni 1 wt% Gr/Ni 2 wt% Gr/Ni 3 wt% Gr/Ni 4 wt% Gr/Ni 5 wt% Ni Pure

0.0 0.2 0.4 0.6 Applied Voltage (V) (vs. Ac

Ag/AgCl)

Fig. 6: CV plots of Gr/Ni with different concentrations of nickel (0, 1, 2, 3, 4 and 5 wt%) and Ni-pure electrodes in 1 M KOH at a scan rate of 50 mV s-1. Inset figure is the CV plot of Gr/Ni at 0 wt% electrode in 1M KOH at a scan rate of 50 mV s-1.

160 140 „ 120100

~ 80 oo

5 60 T3

H 40 O 20-| 0

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

Applied Voltage (V) (vs. Ag/AgCl)

Fig. 7: CV plots of Gr/Ni with different concentrations of nickel (0, 1, 2, 3, 4 and 5 wt.%) and pure Ni electrodes in 1 M KOH with 0.33 M urea at a scan rate of 50 mV s-1. Inset figure is the relationship between the current density (mA cm-) and the concentration of nickel (wt%) at a potential of 0.9 V.

<n c 0 T3

100 ■ 90 ■ 8070 ■ 60. 50 ■ 4030 ■ 20. 10 0 -10-20 ■

1M KOH

1M K0H+0.33M Urea 1M K0H+1M Urea 1M K0H+2M Urea 1M K0H+3M Urea

0.0 0.2 0.4 0.6 0.8 Applied voltage (V) (vs. Ag/AgCl)

Fig. 8: CV plots of Gr/Ni (3 wt%) electrode in 1 M KOH with different concentrations of urea (0, 0.33, 1, 2 and 3 M) at a scan rate of 50 mVs-1.

0 20 40 60

-■--1-'-r

30 100 120 140 160 180

time (min)

Fig. 9: Chronoamperometry plots of Gr/Ni (3 wt%) electrode in 1 M KOH, with 0.33 M urea at an applied voltage of 0.5 V.

0.20 0.180.160.14 ■

£ 0.12. 03

O 0.10 ■ >

0.08 -

o 0.060.040.02 0.00

—■— Cell Voltage —• — Power Density

1 2 3 4 5

Current Density (mA.cm"2) x 10-2

Fig.10: Polarization and power density curves of DUFC using Gr/Ni (3 wt%) as anode catalyst and Pt/C (20%) as cathode catalyst.

Fig.11: The relationship between (

Fig. 12 Influence of the current density on the instant internal resistance of the assembled

Table 1: Electroactive surface areas of pure Ni and Gr/Ni at 1, 2, 3, 4 and 5 wt%.

Sample Gr/Ni Gr/Ni Gr/Ni Gr/Ni Gr/Ni Pure 5 wt% Ni

1 wt% 2 wt% 3 wt% 4 wt%

ESA (cm2.mg-1) 23.6 21.5 35.8 44.3 27.7 12.4

Table: 2 Comparison between the introduced decorated graphene and some recently reported materials

Electrode

NiMn-CNFs

NiCo(OH)2

Ni electrode

Electrolyte

1 M KOH

5 M KOH

5 M KOH

Urea solution concentration

0.33 M

0.33 M

Maximum

current

(mA/cm )

Generated

Power (mW/cm2)

(Baral ( 20

;arakat et al.

(Yan et al. 2012b)

(Vedharathinam and Botte 2012)

NiOH nanoribbons

5 M KOH

0.33 M

(Wang et al. 2012)

5 M KOH

0.33 M

(Yan et al. 2012a)

Ni-Zn-Co

5 M KOH

0.33 M

(Yan et al. 2012a)

0.2 (Pt/C cathode; Modified membrane

(Lan et al. 2010a)

GO-Ni :

1 M KOH

Ionic liquid-

Ni(II)-graphite -

Ni-decorated graphene

0.33 M

0.09 (Ag/C anode and Modified membrane

(Lan et al. 2010a)

(Wang et al. 2013)

1 M NaOH

(Chen et al. 2015)

1 M KOH

0.33 M

4 (Pt/C cathode, commercial

cation membrane

This study