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Hierarchical Ni(OH)/polypyrrole/graphene oxide nanosheets as excellent electrocatalysts for the oxidation of urea Zhenqian Cao, Hui Mao, Xi Guo, Dayin Sun, Zhijia Sun, Baoxin Wang, Yu Zhang, and Xi-Ming Song ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04027 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018

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ACS Sustainable Chemistry & Engineering

Hierarchical Ni(OH)2/polypyrrole/graphene oxide nanosheets as excellent electrocatalysts for the oxidation of urea Zhenqian Cao, Hui Mao*, Xi Guo, Dayin Sun, Zhijia Sun, Baoxin Wang, Yu Zhang and Xi-Ming Song* Liaoning Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, College of Chemistry, Liaoning University, Chongshan Middle Road, No. 66, Shenyang 110036, China *

Author to whom any correspondence should be addressed.

Tel.: 86-24-62202378; Fax: 86-24-62202380; E-mail: [email protected] (H . M.); [email protected] (X.M.S)

ABSTRACT: Two kinds of Ni(OH)2 nanostructures, including thin nanosheets (NSs) with the thickness of 15-20 nm and very small nanoparticles (NPs) with the diameter of 2-3 nm, were immobilized on polypyrrole/graphene oxide (PPy/GO) via a facile strategy due to the coordination interaction between Ni2+ and -NH- segments in PPy chains. The obtained Ni(OH)2/PPy/GO presented excellent electrocatalytic activity for urea oxidation reaction (UOR) due to the excellent hierarchical nanostructures and the synergistic effects of lamellar GO, conductive PPy, electrocatalytically active Ni(OH)2. The electrocatalytic mechanism and the role of the each component in the Ni(OH)2/PPy/GO to UOR were also investigated in detail. PPy/GO with good conductivity and large surface area can effectively facilitate the electronic transmission of UOR. Super smaller Ni(OH)2 NPs are electrocatalytic active centers for UOR. Therefore, Ni(OH)2/PPy/GO can serve as a kind of excellent electrocatalysts for UOR in alkaline medium and reveal potential applications to urea-rich wastewater disposal, hydrogen production and direct urea fuel cells (DUFCs).

KEYWORD: nickel hydroxide; polypyrrole/graphene oxide (PPy/GO); urea; electrocatalytic oxidation.

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INTRODUCTION In the recent decades, energy crisis and environment pollution have attracted world-wide attention and driven researchers to explore and develop efficient, durable, sustainable and clean energy sources independent of fossil fuels.1, 2 Urea is a kind of widely used nitrogen-release fertilizer in the agricultural industry, but large-scale discharge of urea-rich wastewater which derives from urea synthesis industry and urine waste excreted from human/animal often pollutes the air, the ground and drinking water.3, 4 Recently, urea electrolysis, whose reaction equation is CO(NH2)2 + H2O → N2 + 3H2 + CO2, has attracted tremendous attentions, because it is a broad applied and effective technique approach for purifying urea-rich wastewater, simultaneously producing hydrogen at the cathode,5 which barely need sophisticated and bulky instruments.6 Therefore, direct urea fuel cells (DUFCs) offer a great promising application for energy-sustainable development and mitigating water contamination. The equation of anodic urea-oxidation reaction (UOR) is CO(NH2)2 + 6OH- → N2 + 5H2O + CO2 + 6e-,7, 8 which results in the require of catalysts based on noble metal for UOR, including Pt3 and Ru.9 However, the resource scarcity and the high price of these catalysts hinder the commercialization of the urea electrolysis. Up to now, considerable efforts have been attracted on developing low-cost alternatives with high catalytic performance to noble-metal catalysts.10 For instance, Ni can act as the superior active species for UOR in alkaline medium,3 but the practical application of pure Ni catalysts was hindered by their disadvantages, such as low electroactive sites and easy CO-poisoning behavior usually.2 Recently, inexpensive Ni-based catalysts, including Ni hydroxides/oxides10 or other compounds8 have been developed for UOR. Some researches suggest that NiOOH can be produced during electrocatalytic process of UOR, because that its electroactive sites and anti-poisoning property are highly corresponding to the structural/electronic effects of Ni-based catalysts.11, 12 Therefore, for improving the electroactivity and stability of Ni-based electrocatalysts, their physical/chemical structures would be tuned by preparing supported-type nanocatalysts, which are composed of effective catalytic active sites with fine particle/pore size and appropriate supporters for promoting electronic transmission of UOR.


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Since discovered in 1977,13 conducting polymers possessing high conductivity, redox properties and excellent environmental stability, have attracted many attentions for promising applications in various areas.14,

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Conducting polymer micro/nanostructures can be selected as good catalyst supports for

preparing supported-type nanocatalysts, because transition metal compounds can be well distributed on their surface because of the coordination interaction between transition metal ions and the heteroatoms in conducting polymer chains. For instance, good electrocatalytic activities can be achieved by PPyNiOx composite film through electrosynthesis in the application of ethanol oxidation in alkaline media;16 a ternary composite composed of transition metal hydroxides, PPy and reduced graphene oxide exhibited excellent bifunctional electrocatalytic activities and promising electrochemical durability for the oxygen reduction and the oxygen evolution reactions in alkaline solution;17 a non-enzymatic glucose sensor can be constructed by the hierarchical nanostructure of PPy nanowires electrode modified with Ni(OH)2 nanoflakes.18 Therefore, the intrinsic electric conduction characteristics of conducting polymer micro/nanostructures19 may enhance the electrocatalytic performance of the obtained supported-type nanocatalysts. In this work, two kinds of Ni(OH)2 nanostructures, including thin nanosheets (NSs) with the thickness of 15-20 nm and very small nanoparticles (NPs) with the diameter of 2-3 nm, were successfully immobilized on polypyrrole/graphene oxide (PPy/GO) by the coordination interaction between Ni2+ and -NH- segments in PPy chains. Through this facile strategy, Ni(OH)2/PPy/GO nanosheets were synthesized. The excellent electrocatalytic activity for UOR was achieved by Ni(OH)2/PPy/GO modified glassy carbon electrode (Ni(OH)2/PPy/GO/GCE) due to the excellent hierarchical nanostructures and the synergistic effects of lamellar GO, conductive PPy, electrocatalytically active Ni(OH)2, especially the good conductivity of PPy can effectively facilitate the electronic transmission of UOR. The electrocatalytic mechanism and the role of the each component in the Ni(OH)2/PPy/GO to UOR were also investigated in detail, indicating that Ni(OH)2/PPy/GO can act as a kind of excellent electrocatalysts for UOR in alkaline media and reveal potential applications to urea-rich wastewater disposal, hydrogen production and direct urea fuel cells (DUFCs). ACS Paragon Plus Environment

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EXPERIMENTAL SECTION Preparation of Ni(OH)2/PPy/GO nanosheets 10 mg of PPy/GO (synthesized as the previous report20) and 14 mL deionized water were under ultrasound for 30 min. Ni(NO3)2 solution (14 mL, 5 mM) and NaSH solution (14 mL, 15 mM) were added into PPy/GO suspension, kept at 60 °C for 6 h. The cooled products were washed by water and ethanol for several times, drying in vacuum at 50 °C for 24 h. The synthetic process of Ni(OH)2/PPy/GO nanosheets is showed in Scheme 1. In addition, pure Ni(OH)2 and Ni(OH)2/GO nanosheets were also fabricated by the methods given in the Supporting Information, whose morphologies were also presented in Fig. S1.

Scheme 1. The synthetic process of Ni(OH)2/PPy/GO nanosheets.

Preparation of Ni(OH)2/PPy/GO/GCE ACS Paragon Plus Environment

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1 mg of Ni(OH)2/PPy/GO nanosheets were dispersed into 1 mL ethanol for the preparation of Ni(OH)2/PPy/GO suspension. 5 µL suspension were dropped on a GCE (φ = 3 mm) to form a layer and drying in the environment. The working electrode is the above modified GCE.

RESULTS AND DISCUSSION Characterizations of Ni(OH)2/PPy/GO

Fig. 1. XRD patterns of (a) GO, (b) PPy/GO, (c) Ni(OH)2 and (d) Ni(OH)2/PPy/GO.

XRD patterns of GO, PPy/GO, Ni(OH)2 and Ni(OH)2/PPy/GO are shows in Fig. 1. The characteristic peak of GO in Fig. 1(a) was clearly found at 2θ = 11.6° with the (001) interlayer spacing of 0.78 nm， which had been attributed to the existence of hydroxyl, epoxy and carboxyl groups.21 A small characteristic peak at 2θ = 42.4° was obtained from the (001) plane of graphite phases.22 A broad peak center at 2θ = 25° was only observed in the XRD pattern of PPy/GO in Fig. 1(b), which belonged to the ACS Paragon Plus Environment

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characteristic diffraction peak of amorphous PPy.22, 23 The result means that GO nanosheets were coated by PPy nanosheets with electrostatic interactions and π− π stacking between them. The homogeneity and crystallinity of Ni(OH)2 were determined by XRD in Fig. 1(c), and all the diffraction peaks could be matched well with hexagonal β-Ni(OH)2 (JCPDS file number 14-0117, signal

●

).24 Both the

characteristic peaks which 2θ = 25° of PPy/GO and the characteristic peaks of hexagonal β-Ni(OH)2 could be found in Fig. 1(d), which testified the existence of PPy/GO and Ni(OH)2.

Fig. 2. SEM images of (a) PPy/GO, (b) Ni(OH)2/PPy/GO, TEM image of (c) Ni(OH)2/PPy/GO and HRTEM of (d) Ni(OH)2/PPy/GO.


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SEM and TEM were used for the investigation of the morphology of Ni(OH)2/PPy/GO nanosheets in Fig. 2. Compared to PPy/GO in Fig. 2(a), many smaller NSs and NPs can be clearly observed on Ni(OH)2/PPy/GO in Fig. 2(b), indicating that Ni(OH)2 nanostructures were successfully immobilized on PPy/GO because of the coordination interaction between Ni2+ and -NH- segments in PPy chains. The typical TEM image of Ni(OH)2/PPy/GO nanosheets is presented in Fig. 2(c). It is clearly found that inorganic lamellar structures with the thickness of 15-20 nm and very small NPs with the diameter of 23 nm existed on PPy/GO. A high-resolution TEM (HRTEM) image of Ni(OH)2/PPy/GO clearly presents the lattice fringes corresponded to the highly crystalline nature of Ni(OH)2 in Fig. 2(d). The lattice spacing is measured to 0.231 nm, matching well with (101) interplaner spacing of β-Ni(OH)2 (JCPDS file number 14-0117), which well coincided with Fig. 1(d). In addition, from the XRD data, it is worth to note that compared to the peak at 2θ = 33.2°, the peak at 2θ = 38.4° was obviously widened. According to Scherrer formula,25 the average sizes estimated from the diffraction peaks of (100) and (101) plane were calculated to 18.5 nm and 2.5 nm, respectively, consisting with the average thickness of Ni(OH)2 NSs and the average diameter of Ni(OH)2 NPs observed from TEM image of Ni(OH)2/PPy/GO in Fig. 2(c).


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Fig. 3. FTIR spectra of (a) GO, (b) PPy/GO, (c) Ni(OH)2 and (d) Ni(OH)2/PPy/GO.

FTIR spectra are shown in Fig. 3 to confirm the chemical structures of Ni(OH)2/PPy/GO. The characteristic peaks of all samples are identified and assigned are presented in Table 1, confirming that the final product is composed of Ni(OH)2, PPy and GO.

Table 1. vibration modes and band frequencies in GO, PPy/GO, Ni(OH)2 and Ni(OH)2/PPy/GO.

Species

Peak (cm-1)

GO

3421

-OH stretching vibrations

3421

C-H stretching vibrations

Functional group and vibration

References 29, 30, 31, 32


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PPy/GO

1722

carboxyl C=O stretching vibrations

1623

aromatic C=C skeletal vibrations

1046

alkoxy C-O deformation vibrations

1542

C-C antisymmetric vibration in PPy rings

26

1288

=C-N in-plane stretching vibration in PPy rings

27

1175, 789

Ni(OH)2

C-C stretching vibrations

35, 36, 37

900

the doping state of PPy

3642

O-H vibration of the free non-hydrogen bonded hydroxyl group

28

1637

O-H rocking vibration

29

523, 475

the deformation mode of Ni-OH stretching vibrations modes

40, 41

The EDS spectra of Ni(OH)2 and Ni(OH)2/PPy/GO coated on a silicon pellet are showed in Fig. S2, which can further certify the chemical composition of Ni(OH)2/PPy/GO. Only the peaks of Ni and O element (Si pellet as substrate) can be found in Fig. S2(i). The peaks of C and N can be also clearly found in Fig. S2(ii), which further testify that Ni(OH)2 have been immobilized on PPy/GO. However, an additional peak of S element appears in Fig. 4(ii), which may be due to the residual S from the postprocessing.


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Fig. 4. XPS spectra of Ni(OH)2/PPy/GO: (i) survey spectra; (ii) C 1s; (iii) N 1s; (iv) O 1s; (v) Ni 2p.


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XPS analysis was performed to further determine the chemical states of Ni(OH)2/PPy/GO, which more clearly demonstrated the evidence for the coordination between Ni2+ and –NH- group. Fig.4(i) shows the characteristic signature for C, O, N and Ni element in the survey scan spectrum of Ni(OH)2/PPy/GO and their core-line spectra are presented in Fig.4(ii)-(v), respectively, and Table 2 displays the corresponding bonding energies and attribution, which can well confirm the existence of Ni(OH)2, PPy and GO in product, as well as the coordination between Ni2+ and –NH- group.

Table 2. binding energies and assignment for the fit of C 1s, N 1s, O 1s and Ni 2p spectra of Ni(OH)2/PPy/GO

Attribution

Element

B. E. (eV)

C 1s

283.6

sp2-hybridized carbon

30

284.8

C-C/C-H,

31

286.2

C-N+ of Py rings

32

287.7

C=N+ of Py rings

32

289.0

O-C=O of Py rings

33, 34

397.9

C=N defects of PPy and the coordination of N-Ni

35, 36

399.1

-NH- group of pyrrole unit

37

400.0

-N+H- group of pyrrole unit

35

530.9

the bound hydroxide groups (OH-)

38

532.9

the oxygen of the carboxylate (O-C=O) in PPy/GO

39

N 1s

O 1s

References


11


Ni 2p

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40

855.8

Ni 2p3/2

873.4

Ni 2p1/2

861.4

the shake-up satellite

879.4

the shake-up satellite

866.2

the second shake-up satellite of Ni 2p3/2

41

42

Electrochemical behavior of Ni(OH)2/PPy/GO/GCE

Fig. 5. Linear sweep voltammograms of the six modified GCEs a-f in 1 M KOH with scanning rate at 10 mV/s. ACS Paragon Plus Environment

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Fig. 5 presents that the electrochemical behavior of Ni(OH)2/PPy/GO/GCE was investigated by LSV, compared with the other five modified GCEs. Only Ni(OH)2/PPy/GO/GCE exhibits an excellent anodic peak at 0.5 V in Fig. 5(f), implying the higher electrochemical activity because of the oxidation of Ni(OH)2 to the nickel oxyhydroxide (NiOOH).43

Fig. 6. (i) Cyclic voltammograms of Ni(OH)2/PPy/GO/GCE at different scanning rates in 1 M KOH; (ii) plots of jpa vs. scanning rates from (i); (iii) Laviron's plots of (a) anodic and (b) cathodic peak potential vs. ln ν from (i).


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Fig. 6(i) present the influence of scanning rates evaluated using CV. A pair of enhanced redox peaks in 1 M KOH solution can be observed, corresponding to Ni(II)/Ni(III) redox couple.44 The oxidation peak current density (jpa) of Ni(II)/Ni(III) redox couple at Ni(OH)2/PPy/GO/GCE increased continuously with the scanning rates. jpa value presented an excellent linear relationship against ν0.5 in the range from 10 to 100 mV/s (R2 = 0.9995) in Fig. 6(ii), indicating that the electrochemical characteristic of the diffusion process of OH- from the solution to Ni(OH)2/PPy/GO/GCE was a typical diffusion-controlled process.45 In addition, by the increasing of scanning rates, the oxidation peak potential (Epa) on Ni(OH)2/PPy/GO/GCE moved in the direction of the positive potential, and the corresponding reduction peak potential (Epc) moved in the direction of negative potential, resulting in the gradual increasing of the difference of anodic peak-to-cathodic peak position (∆Ep). Fig. 6(iii) shows that both of Epa and Epc depended linearly on ln ν (10 to 100 mV/s) with the equations of Epa (V) = 0.5819 + 0.0189 ln ν (V/s) (R2 = 0.9595) and Epc (V) = 0.333 - 0.0069 ln ν (V/s) (R2 = 0.9258), respectively. Hence, according to the Laviron theory for thin-layer quasi-reversible electrochemical process,46 the electrochemical parameters of the electron transfer coefficient α and the apparent charge transfer rate constant ks can be defined by the following equations: Epa = E o +

RT ln ν (1− α )nF

Eq. (1)

Epc = E o −

RT ln ν αnF

Eq. (2)

And ln ks = α ln (1 − α ) + (1 − α ) ln α − ln

RT α (1 − α )nF∆Fp − nFν RT

Eq. (3)

Where n is the number of transferred species, ν is scanning rate and other symbols have their usual meanings. The electron transfer coefficient α was calculated to 0.733 according to Eq. (1) and Eq. (2), and the apparent charge transfer rates constant ks was calculated to 2.21 s−1 according to Eq. (3). Otherwise, according to the previous reports,47,

48

the electroactive surface area (ESA) of the

Ni(OH)2/PPy/GO/GCE could be also estimated by the following equation: ACS Paragon Plus Environment

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ESA =

Q mq

Eq. (4)

Where Q is the charge for hydrogen desorption (mC/cm2), which could be calculated from CV in Fig. 6(i) of 10 mV/s, m is the quantity of nickel element and q is 246 µC/cm2,49 meanwhile, the proportion of Ni element in Ni(OH)2/PPy/GO was 24.16% measured by ICP measurement, so that the ESA of Ni(OH)2/PPy/GO was calculated to 116 cm2/mg, which was much higher than that of Ni(OH)2 nanoribbon (2.1 cm2/mg),48 Ni-Zn catalysts (67.9 cm2/mg),50 nickel nanowire (79.1 cm2/mg),51 etc., presented in Table 3.

Table 3. ESA for different Ni based electrocatalysts estimated from the CV curves presented in some published papers.

Materials

ESA (cm2/mg)

References

Ni(OH)2 nanoribbon

2.1

48

Ni-Zn catalysts

67.9

50

Nickel nanowire

79.1

51

Ni(OH)2/PPy/GO

116

This work

Electrocatalytic oxidation of urea at Ni(OH)2/PPy/GO/GCE


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Fig. 7. (i) Linear sweep voltammograms of Ni(OH)2/PPy/GO/GCE in 1 M KOH (a) in the absence and (b) in the presence of 0.5 M urea with scanning rate at 10 mV/s; (ii) Chronoamperograms of Ni(OH)2/PPy/GO/GCE in 1 M KOH solution in the absence (a) and presence (b) of 0.5 M urea. Applied potential was 0.5 V vs. Hg/HgO.


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Scheme 2. The oxidation process of urea on Ni(OH)2/PPy/GO/GCE in 1 M KOH.

The electrocatalytic activity of Ni(OH)2/PPy/GO/GCE for UOR were investigated by LSV. Ni(OH)2/PPy/GO/GCE appeared a well-defined oxidation peak derived from Ni(OH)2 to NiOOH at 0.5 V in Fig. 7(i)-a. When 0.5 M urea existed, a much stronger oxidation peak appeared at 0.60 V in Fig. 7(i)-b, which was 100 mV more positive than the potential of Ni(II)/Ni(III) conversion at Ni(OH)2/PPy/GO/GCE in the absence of urea. It indicated that the electrooxidation of urea occurred after Ni(II) was oxidized to Ni(III), where Ni(III) was used as an electrocatalytic center for UOR.52 Fig. 7(ii) displays the chronoamperograms of Ni(OH)2/PPy/GO/GCE in the absent and present of urea with the continuous and constant current density. The higher current density can be obtained in the present of urea, revealing that Ni(OH)2/PPy/GO can serve as an excellent and stable electrocatalyst for UOR in alkaline solution. Therefore, the oxidation process of urea on Ni(OH)2/PPy/GO/GCE can be described in ACS Paragon Plus Environment

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Scheme 2, and the potential mechanism for UOR process can be explained as follows: (i) the oxidation process of Ni(II)/Ni(III), the oxidation of Ni(OH)2 to NiOOH occurred fast on Ni(OH)2/PPy/GO/GCE by the power source, as described in Eq. (5); (ii) electrooxidation of urea, urea was oxidized to the product by Ni(III) and this process was much slower than that of Ni(II)/Ni(III) on Ni(OH)2/PPy/GO/GCE,

where Ni(III) was simultaneously reduced to Ni(II),2, 65, 66 as shown in Eq. (6); (iii) total

reaction for UOR, urea was electrooxidation to N2 and CO32- and H2O in alkaline solution, as summarized in Eq. (7).

Fig. 8. Linear sweep voltammograms of the six modified GCEs a-f in 1 M KOH containing 0.5 M urea with scanning rate at 10 mV/s.

The catalytic performance of Ni(OH)2/PPy/GO for UOR was markedly higher than that of the above other five materials modified GCE in Fig. 8. No peaks can be found in Fig. 8(a) obtained by the bare ACS Paragon Plus Environment

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GCE. It can be obviously found that GO/GCE in Fig. 8(b) showed a weaker anodic peak for UOR than Ni(OH)2/GO/GCE in Fig. 8(e), which well demonstrated that Ni(OH)2 can serve as electrocatalytic active centers for UOR. However, Ni(OH)2/GCE exhibited poor electrocatalytic activity in Fig. 8(d) because of the bad electroconductivity of Ni(OH)2, which impeded the electron transport on the modified electrode. Though GO/GCE (Fig. 8(b)) exhibited higher electrocatalytic activity than PPy/GO/GCE (Fig. 8(c)) because of the existence of abundant oxygen-containing functional groups of GO, the excellent electrocatalytic activity for UOR was still achieved by Ni(OH)2/PPy/GO/GCE (Fig. 8(f)) compared to Ni(OH)2/GO/GCE (Fig. 8(e)), which may be associated with more Ni(OH)2, especially super smaller Ni(OH)2 NPs immobilized on PPy/GO by the coordination interaction between Ni2+ and -NH- segments in PPy chains and the good conductivity of PPy which can effectively facilitate the electronic transmission of UOR. Based on the above discussion, in our opinion, the role of the each component in the Ni(OH)2/PPy/GO to UOR may be explained as follows: lamellar GO provided large surface area; conductive PPy can not only effectively facilitate the electronic transmission of UOR, but also contribute to the immobilization of Ni(OH)2 nanostructures on PPy/GO by the coordination interaction between Ni2+ and -NH- segments in PPy chains, which result in the excellent hierarchical nanostructures of the obtained Ni(OH)2/PPy/GO nanosheets; Ni(OH)2 nanostructures, especially super smaller Ni(OH)2 NPs can serve as electrocatalytic active centers for UOR. Meanwhile, the excellent hierarchical structures between Ni(OH)2 nanostructures (including thin NSs and small NPs) and PPy/GO nanosheets can further effectively improve the electrocatalytic performance of electrocatalysts. Therefore, due to the excellent hierarchical nanostructures and the synergistic effects of lamellar GO, conductive PPy, electrocatalytically active Ni(OH)2, the excellent electrocatalytic activity for UOR was achieved by Ni(OH)2/PPy/GO/GCE.


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Fig. 9. (i) Linear sweep voltammograms of Ni(OH)2/PPy/GO/GCE at different scanning rates in 1 M KOH with 0.5 M urea; (ii) plots of potential vs. log ν from (i); (iii) plots of jpa for urea vs. scanning rates from (i) and (iv) Tafel plots of Ni(OH)2/PPy/GO in 1 M KOH with 0.5 M urea.

Fig. 9(i) presents the effect of scanning rates on UOR at Ni(OH)2/PPy/GO/GCE evaluated by LSV. Well-defined oxidation peaks of urea can be obtained at different scanning rates, and jpa increased proportionally. The plots of jpa for urea against the scanning rate shows an excellent linear relationship in Fig. 9(ii), whose linear regression equation is IUOR (mA/cm2) = 0.0686 + 0.4427 ν0.5 (R2 = 0.9976), demonstrating that UOR at Ni(OH)2/PPy/GO/GCE was a typical diffusion-controlled process.67 ACS Paragon Plus Environment

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Simultaneously, Epa depended linearly on log ν in Fig. 9(iii), which demonstrated a kinetic limitations on UOR.53 The diffusion coefficient of urea (Durea) can be calculated by Eq. (8): Ipa = 2.99 × 105 n[(1 − αUOR )n 0] ACDurea 0.5ν 0.5 0 .5

Eq. (8)

The symbols of above formula are consist with those in the previous report.1 According to the previous report,53 the value of αUOR can be calculated based on Fig. 9(iii) using the linear dependency of Epa with log ν by Eq. (9): Epa = k +

0.03 log ν αUORn 0

Eq. (9)

where k is a constant. Hence, αUOR was calculated to 0.946 by the slope of Fig. 9.(iii) and after substituted into Eq. (e), Durea of 9.2 × 10-10 cm2/s was calculated from Fig. 9(ii) for the electrochemical UOR on Ni(OH)2/PPy/GO/GCE. Fig. 9(iv) depicted the Tafel slope of Ni(OH)2/PPy/GO/GCE for UOR in the kinetics range (0.47-0.50 V) was calculated to 30.9 mV/dec, which was lower than that of rNiMoO4/nickel foam (32 mV/dec),54 mesoporous nickel phosphide nanocatalysts (52 mV/dec),12 smallsized MnO2 nanocrystals (75 mV/dec),55 etc., presented in Table 4.

Table 4. Tafel slopes for various Ni based electrocatalysts estimated from the LSV curves presented in some published papers.

Materials

Scanning rate (mV/s)

Estimated Tafel slopes (mV/dec)

r-NiMoO4/NF a

5

32

54

Mesoporous nickel phosphide nanocrystals

1

52

12

Small-sized MnO2 nanocrystals

-

75

55

Ni(OH)2/NiOOH b

2

44

56

References


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Ni8Mo1/G c

1

110

2

Ni(OH)2/PPy/GO

5

30.9

This work

a

Oxygen-vacancies rich NiMoO4 grown on Ni foam substrates

b

Electrodeposition of Ni(OH)2/NiOOH catalyst on GCE

c

Nickel-molybdenum/graphene nanocatalysts


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Fig. 10. (i) Linear sweep voltammograms of Ni(OH)2/PPy/GO/GCE at a scanning rate of 5 mV/s in different [KOH] with 0.5 M urea; (ii) the corresponding variation in jpa of UOR with different [KOH] with 0.5 M urea (data taken from (i)); (iii) Tafel plots at various [KOH] (0.5-4 M) in 0.5 M urea solution at a scanning rate of 5 mV/s; (iv) double logarithmic plot of j as a function of [KOH] at constant electrode potentials: (a) 450, (b) 455, (c) 460 V, conditions as in Fig. 10(iii).

In addition, the effects of the concentration of KOH and urea ([KOH]) and [urea]) on UOR at Ni(OH)2/PPy/GO/GCE

were

also

investigated.

Fig.10(i)

presents

the

LSV

curves

by

Ni(OH)2/PPy/GO/GCE in different [KOH] and the corresponding variation of jpa of UOR is shown in Fig. 10(ii), where an improvement in jpa with increasing OH- concentration can be observed. Along with the increasing of [KOH] in the range of 0.01-3 M, jpa of UOR gradually enhanced, but when [KOH] was higher than 3 M, jpa of UOR remained almost constant. jpa was linearly depended on [KOH] in the range of 0.1-2 M and deviated from linearity up to 2 M KOH, exhibiting a diffusion limitation of the OH- to the electrocatalyst surface on GCE. It may be mainly attributed to the complete coverage of the electrocatalyst surface with OH-, which resulted in impeding the mutual contact between urea molecules and Ni(OH)2/PPy/GO on GCE for further oxidation.1 Fig. 10(iii) presents Tafel plots at various [KOH] (0.5-4 M), revealing that the onset potential can be improved by the increasing OH- concentration.1 Otherwise, the rate of UOR also increased along with the increasing of OH- concentration in the potential region of 0.35-0.5 V, indicating that the high OH- concentration enhanced the rate of UOR.57 Fig. 10(iv) displays the linear relationship between log ｜ j ｜ and log [KOH] at constant electrode potential. The slope of all tested potentials is approximate to 2, suggesting the reaction order is 2 related to OH- ions.1


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Fig. 11. (i) Linear sweep voltammograms of Ni(OH)2/PPy/GO/GCE at a scanning rate of 5 mV/s in 1 M KOH with different [urea]; (ii) the change in anodic peak current density at various [urea] from (i).

Fig. 11 presents the effect of [urea] on UOR at Ni(OH)2/PPy/GO/GCE, which was investigated by LSV. jpa of UOR gradually enhanced with the increasing of [urea] in the range of 0.1-1.5 M. It can be found that jpa deviated from linearity at higher [urea] and Epa related to UOR increased with the increasing of [urea] over the range of 0.1-1.5 M, which may be due to the competition between the surface coverage of urea molecules and the adsorbed OH- coverage on the electrocatalyst surface at GCE, consist with the previous report1.

Electrochemical impedance analysis


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Fig. 12. EIS plots of (a) GO/GCE, (b) PPy/GO/GCE, (c) Ni(OH)2/GCE and (d) Ni(OH)2/PPy/GO/GCE in 0.1 M KCl solution containing 2.5 mM K3Fe(CN)6 and 2.5 mM K4Fe(CN)6.

The Nyquist diagrams of GO/GCE, PPy/GO/GCE, Ni(OH)2/GCE and Ni(OH)2/PPy/GO/GCE measured by EIS. Based on the literature,58 a semicircle portion in EIS plots denoted an electro-transfer resistance (Rct), delegating the electron transfer kinetics of the redox probe at the electrode interface. The Niquist diagrams of GO/GCE and PPy/GO/GCE were consistent with previous report.59 The slope of PPy/GO/GCE in Fig. 12(b) showed more dramatic than that of GO/GCE (Fig. 12(a)), indicating the higher electron conduction pathways. The Nyqust diagram of Ni(OH)2/GCE in Fig. 12(c) presents the largest diameter of the impedance arc among all the modified GCEs, which demonstrated Ni(OH)2/GCE possessed the highest interfacial Rct. It is clearly found that the diameter of the impedance arc obtained by Ni(OH)2/PPy/GO/GCE in Fig. 12(d) was distinctly smaller than Ni(OH)2/GCE, implying the lowest Rct. The Rct of the GO/GCE, PPy/GO/GCE, Ni(OH)2/GCE and Ni(OH)2/PPy/GO/GCE is 1185, 831, ACS Paragon Plus Environment

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4597 and 1989 Ω, respectively. It indicated that the excellent hierarchical Ni(OH)2/PPy/GO and good conductive PPy/GO were conducive to promote the electrons transfer of Ni(II)/Ni(III) redox conversion on the electrode interface, which further demonstrated the excellent electrocatalytic activity of Ni(OH)2/PPy/GO/GCE for UOR.

CONCLUSIONS In general, novel hierarchical Ni(OH)2/PPy/GO nanosheets were prepared via a facile strategy. The Ni(OH)2/PPy/GO/GCE presented the excellent electrocatalytically activity for UO and Tafel slope was calculated to 30.9 mV/dec, because of the excellent hierarchical nanostructures and the synergistic effects of the lamellar GO, conductive PPy, electrocatalytically active Ni(OH)2. The electrocatalytic mechanism of UOR on Ni(OH)2/PPy/GO/GCE have been investigated in detail, where urea were oxidized by Ni(III) on the electrode surface and PPy/GO with good conductivity and large surface area could effectively facilitate the electronic transmission of UOR. Therefore, the Ni(OH)2/PPy/GO nanosheets could serve as good electrocatalysts for UOR in alkaline medium and reveal promising applications to urea-rich wastewater disposal, hydrogen production and DUFCs.

Supporting Information SEM images of (a) Ni(OH)2 and (b) Ni(OH)2/GO EDS spectra of (i) Ni(OH)2 and (ii) Ni(OH)2/PPy/GO

Acknowledgements The financial supports from the National Natural Science Foundation of China (No. 51203072, 51773085 and 21203082) are greatly appreciated.


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Hierarchical Ni(OH)2/PPy/GO nanosheets exhibited the excellent electrocatalytic activity towards urea oxidation reaction. 82x31mm (300 x 300 DPI)


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