Highly Dispersed Ag-Functionalized Graphene Electrocatalyst for

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Highly Dispersed Ag-Functionalized Graphene Electrocatalyst for Oxygen Reduction Reaction in Energy-saving Electrolysis of Sodium Carbonate Bao Men, Yanzhi Sun, Yang Tang, Linying Zhang, Yongmei Chen, Pingyu Wan, and Junqing Pan Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 08 Jul 2015 Downloaded from http://pubs.acs.org on July 9, 2015

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Industrial & Engineering Chemistry Research

Highly Dispersed Ag-Functionalized Graphene Electrocatalyst for Oxygen Reduction Reaction in Energy-saving Electrolysis of Sodium Carbonate

Bao Men, Yanzhi Sun*, Yang Tang, Linying Zhang, Yongmei Chen, Pingyu Wan* and Junqing Pan National Fundamental Research Laboratory of New Hazardous Chemicals Assessment and Accident Analysis, Institute of Applied Electrochemistry, Beijing University of Chemical Technology, Beijing 100029, China.

AUTHOR INFORMATION

Corresponding Author * Tel./Fax: +86 10 64435452. E-mails: [email protected]; [email protected]. edu.cn

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ABSTRACT: In this paper, Ag-Functionalized graphene electrocatalyst was prepared

by simultaneous reduction of Ag[(NH3)2]+ and graphene oxide (GO) under the protection of Poly Diallyldimethylammonium Chloride (PDDA). The as-prepared catalyst was utilized to enhance the catalytic activity towards oxygen reduction reaction (ORR) in energy-saving electrolysis of Na2CO3. The morphology characterization indicates that Ag nanoparticles uniformly disperse on the surface of reduced graphene oxide (RGO) and their average size is only about 5.7 nm. The electrochemical tests show the as-prepared catalyst exhibits high electrocatlytic activity for ORR in alkaline media. Furthermore, when the catalyst is used for the oxygen reduction cathode (ORC) in the galvanostatic electrolysis of Na2CO3, the cell voltage can be reduced by 1.05 V as compared with the conventional hydrogen evolution cathode (HEC) electrolysis. Correspondingly, up to 41.5% electrical energy consumption is saved at the same current density of 100 mA cm−2.

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1. INTRODUCTION In the recent decades, ORR has been paid much attention to because it has become one of the most important electrochemical reactions for fuel cells and metal-air batteries.1-3 ORR has also been applied to the electrolytic production such as the electrolysis of chlorate, chlor-alkali and sodium carbonate (Na2CO3) due to its enormous energy-saving potentiality.4-6 In our previous work, we reported an efficient and sustainable technique to produce alumina by electrolysis of Na2CO3.7 The new technique can greatly improve the productivity of the conventional Bayer process in alumina industry. Nevertheless, the cell voltage for the electrolysis of Na2CO3 is as high as 2.53 V, resulting in a relatively high electricity consumption. The main reason is that the corresponding cathodic process is hydrogen evolution reaction (HER) of rather negative potential (Eq. 1). If the ORR via a direct four-electron transfer pathway (Eq. 2) is used to replace the cathode HER, the cell voltage will be reduced by 1.23 V theoretically, and thus an energy saving of approximate 40 % will be achieved.6 Cathode 1 : 2H2O + 2Na+ + 2e− → 2NaOH + H2

Eθ= - 0.83 V

(1)

Cathode 2 : 2H2O + O2 + 4Na+ + 4e− → 4NaOH

Eθ= 0.40 V

(2)

Therefore, looking for an ORR catalyst with high activity is hanging over scientists' heads. To facilitate the activity of ORR, various electrocatalysts have been developed

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over the past few years. Precious metals, especially carbon-supported platinum (Pt) and Pt-based alloy nanoparticles have been demonstrated to be the most excellent catalysts for ORR because of their efficient and high catalytic activity.8,9 However, overdependence on these noble metals results in some limitations in application due to their high price, low storage and poor stability in long-term operation. So it is necessary to develop some inexpensive and stable electrocatalytic materials as substitutes for the Pt-based catalysts.10,11 The carbon-supported non-precious metals and transition metal oxides (Ag/C, CoOx/C, MnOx/C) have been investigated extensively as electrocatalysts to catalyze the ORR in alkaline media.12-14 Among these materials, silver is regarded as the promising candidate to replace Pt because of its relatively high electrochemical activity and long durability towards ORR in alkaline media.15-17 It is known that the properties of the support material have great impact on the activity of the ORR catalysts. Graphene, as two-dimensional (2D) nanosheets composed of few layers of sp2 hybridized carbon atoms, exhibits the intrinsic high conductivity (107 S m-1), high specific surface area (2630 m2 g-1) and high thermal stability.18 These features make it become an outstanding substitute for traditional carbon support materials applied in fuel cells, metal-air batteries and electrochemical sensors. So the Ag/graphene-based materials (Ag/Graphene, Ag-MnO2/graphene and so on) have been synthesized extensively as eletrocatalysts for the ORR in alkaline media.19-21 In spite of the enormous ongoing research for Ag/Graphene electrocatalysts, there are still a lot of great challenges. For instance, the undesired

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reunion through van der Waals interactions22 during the liquid-phase reduction process makes the graphene only afford quite limited accessible area. Another challenge is how to control the size distribution of Ag nanoparticles on graphene because Ag nanoparticles are likely to aggregate due to its high surface energy.23 Although it can be found in a few papers that some surfactant molecules, like PVP, PDDA24 and so on, are used as the modifiers to improve the performance of silver nanoparticles on graphene, there is a rare study that the highly dispersed Ag-graphene electrocatalyst is used as ORC in electrolysis of Na2CO3. In this paper, Ag[(NH3)2]+ and GO were simultaneously reduced at low temperature with the presence of PDDA to avoid the aggregation of Ag nanoparticles and RGO. The synthesized materials were characterized with transmission electron microscopy (TEM), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV-vis absorption spectra. The results show that Ag nanoparticles with average particle size of 5.7 nm extremely uniformly distribute on the sheet-like RGO. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) of rotating disk electrodes show that the as-prepared catalysts exhibit high electrocatlytic activity towards ORR. The synthesized materials were further explored as ORC in the electrolysis of Na2CO3 and up to 41.5% electrical energy consumption was reduced at 100 mA cm-2.

2. EXPERIMENTAL 2.1 Synthesis of Ag-Functionalized graphene nanocomposites. All chemicals in the experiment were analytical grade, purchased from Beijing Chemical Reagents Company, and used as received without further purification. Graphene oxide (GO) 5



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nanosheets were prepared from the natural flake graphite (325 mesh, Aladdin Industrial Corporation) by the modified Hummers’ method.25, 26 The preparation of the Ag-Functionalized graphene nanocomposites consists of the following stages: First of all, 0.1 g mL-1 PDDA (Mw < 100000, 35 wt %, Aladin Ltd., Shanghai, China) was added to 50 mL GO dispersion solution (1 mg mL-1) and the mixture was sonicated for 30 min. Then 1 mL freshly prepared [Ag(NH3)2]+ aqueous solution (0.116 mol L-1) was mixed with the GO solution and stirred for 15 min. Subsequently, 100 mg sodium borohydride (NaBH4) as the reductant was added to the above solution and continuously stirred for 15 min. At last, the well-dispersed suspension was put into a Teflon-lined stainless steel autoclave of 100 mL in volume. The autoclave was sealed and maintained at 80 oC for 24 h. After cooling down to room temperature, the products were separated by centrifugation (5000 rpm) for 30 min, washed with ethanol and deionized water in turn for several times to ensure that PDDA was completely removed from the final product (Figure S1). And then the product was dried in a vacuum oven at 60 oC for 12 h. The obtained product was designated as Ag-PDDA-RGO. For comparison, Ag-RGO and bare functionalized RGO (PDDA-RGO) were prepared using the same hydrothermal method without addition of PDDA and [Ag(NH3)2]+ solution, respectively. 2.2 Structural Characterization. The morphology and granularity of the as-prepared samples were examined by TEM (FEI Technai G2 F20 microscope) and High resolution TEM (HRTEM, JEOL 3010). The XRD measurements were performed by a Rigaku D/max2500VB2+/PCX diffractometer with a Cu anticathode (40 kV, 200

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mA) at a scan rate of 10° min−1 from 5° to 90°. The energy-dispersive X-ray (EDX, Oxford EDS Inca Energy Counter 300, operated at 10 kV) microanalysis was conducted to validate the element compositions and contents. The XPS (ESCALAB250Xi) was performed with a monochromated X-ray source (AlKαhν = 1486.6 eV) to investigate the composition and elemental valence of the catalysts. UV-vis absorption spectra of the synthesized materials were recorded by a Shimadzu UV-visible spectrophotometer (Japan, UV-2550) in a range of 200-700 nm. 2.3 Electrochemical property tests. The rotate disk electrode (RDE) tests were performed with an electrochemistry workstation (PARSTAT 2273) in a three-electrode system at room temperature (25 oC). A platinum wire and Hg/HgO/OH-(0.1M) electrodes were used as the auxiliary electrode and the reference electrode, respectively. 5.0 mg of catalyst powders were dispersed in 970 µL of ethanol plus 30 µL of 0.5 wt % Nafion solution (Du Pont) to form a well-dispersed ink. Then 10 µL of the ink was dropped onto the surface of polished glassy carbon electrode (5.61 mm in diameter) until the solvent was completely evaporated at room temperature, thus the working electrode was loaded with catalyst amount of 0.2 mg cm-2. CV and LSV were carried out to study the electrochemical activity of the samples in ultra-high pure Ar or O2 saturated 0.1 M KOH solution at room temperature. The CV was tested in the potential range from +0.55 to -0.6V vs Hg/HgO/OH-(0.1M). The ORR activity of as-prepared catalyst was evaluated by LSV in oxygen-saturated KOH solution from 0.0 to -0.8 V vs Hg/HgO/OH-(0.1M). The potential of reference electrode was calibrated with the reversible hydrogen electrode (RHE).27 The rotation

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rates of LSV test were 400-2500 rpm. All electrochemical tests were under the scan rate of 10 mV s-1. Prior to each test, the KOH solution was argon or oxygen saturated for 30 min. For comparison, commercial Pt/C (20 wt %) catalyst, Ag-RGO catalyst and PDDA-RGO were also tested by using the same procedure. 2.4 Electrolysis tests. The preparation method of ORC was similar to that reported in our previous work.6 The ORC comprises catalyst layer, gas diffusion layer and current collector. The catalyst layer was prepared as follows: 150 mg as-prepared catalyst and 200 mg acetylene black powders were mixed adequately. Then the mixture and 300 mg polytetrafluoroethylene (PTFE) emulsion (60 wt %) were added to 1.4 mL solution consisting of H2O and C2H5OH (V/V = 1:1) to form solid-liquid mixture paste. The paste was roll-pressed into a 200 µm thick sheet. The sheet was heated up to 150 oC and kept for 40 min, and then cooled to the room temperature naturally. The preparation method of gas diffusion layer was similar to that of the catalyst layer. The gas diffusion layer was prepared with 100 mg ammonium nitrate, 600 mg acetylene black powders and 1200 mg PTFE emulsion, and thermally treated at 320 oC. At last, the catalyst layer, current collector (stainless steel net) and gas diffusion layer were pressed at 10 MPa to form ORC. The prepared ORC and commercial Ti/ RuO2 electrode were used as the cathode and anode of the electrolysis cell, respectively. For conventional HEC electrolysis, the commercial Ni/ RuO2 and Ti/ RuO2 electrodes were used as the cathode and anode, respectively. The effective areas of both electrodes were 8 cm2. The electrolysis cell consisted of two chambers separated by ion-exchange membrane (F6801, Asahi Kasei

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Chemicals Corporation). 1.5 mol L-1 Na2CO3 and 6 mol L-1 NaOH electrolyte were pumped into the anodic and cathodic chambers and circulated through two separate peristaltic pumps, respectively, at a flow rate of 28±2 ml h-1. Oxygen was fed into the gas chamber at a double rate of the theoretical stoichiometry. The current density of electrolysis was controlled at 100 mA cm-2 and the electrolysis temperature was at 70 o

C.

3. RESULTS AND DISCUSSION

In order to research the dispersivity and size of Ag nanoparticles on the layer RGO, the morphologies of as-prepared materials were characterized by TEM. As clearly illustrated in Figure 1a, the flexible sheet-like RGO is transparent with the typical wrinkle structure with the presence of PDDA, which proves that RGO does not obviously stack during the reduction process. This is beneficial to improving the dispersibility of Ag nanoparticles and the properties of RGO. Figure 1b is the TEM picture of Ag-PDDA-RGO, showing a homogeneous dispersion of Ag nanoparticles on the surface of RGO with narrow size distribution. This result benefits from the protection of PDDA and strong complexation between ammonia and Ag+. On the contrary, when PDDA does not exist in the reaction process, it can be seen from Figure 1c that Ag particles unevenly load on stacking RGO with various sizes, implying the occurrence of agglomeration. The EDX of Ag-PDDA-RGO shown in Figure 1d indicates that the loading content of Ag in Ag-PDDA-RGO hybrid is 19.39 wt %, very close to the theoretical addition amount (20 wt %).

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at.%

Element

wt %

C

68.44

85.83

O

12.17

11.46

Ag

19.39

2.71

Figure 1. TEM images of (a) PDDA-RGO, (b) Ag-PDDA-RGO, and (c) Ag-RGO and (d) EDX spectra of Ag-PDDA-RGO.

Figure 2a shows HRTEM image of Ag-PDDA-RGO product and the inset image displays the corresponding particle size distribution histogram. According to inset image of Figure 2a, it is demonstrated that the average diameter of numerous nanoparticles is about 5.7 nm. The HRTEM images taken from one nanoparticle

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shown in Figure 2b reveal clear lattice fringes with interplanar spacing of 0.236 nm and 0.204 nm, corresponding to the (111) and (200) plane of cubic Ag, respectively.28

Figure 2. (a) HRTEM image of the Ag-PDDA-RGO, inset of size distribution histogram of Ag particles, and (b) lattice spacing of Ag nanocrystal of Ag-PDDA-RGO.

Although the preparation mechanism of Ag-PDDA-RGO is still unclear so far, we tentatively speculate the mechanism as shown in Figure 3. It can be seen that without the protection of PDDA, the Ag particles severely aggregate on the surface of stacking RGO. When PDDA is added to the reaction system, positively charged PDDA and Ag[(NH3)2]+ will simultaneously adsorb onto the surface of GO due to the existence of abundant oxygen-containing groups. Then Ag[(NH3)2]+ and GO will be reduced into Ag nanoparticles and RGO nanosheets, respectively. During the reduction procedure, PDDA is expected to act as a stabilizer to keep RGO from restacking through electrostatic repulsion among individual graphene sheets.29 Meanwhile, 11



PDDA can also adsorb the new formed Ag nanoparticles via the long chains and further coat these Ag nanoparticles, avoiding their agglomeration. In addition, the strong complexation between ammonia and Ag+ can effectively decrease the release rate of silver ions, beneficial to the formation of small Ag nanoparticles. This is verified by Figure S2.

COOOH + + + Ag(NH 3)2+ + + NaBH4 + O + OH + OH OH + hydrothermal + HO + + + O + reaction COOAg+ -GO

COO-

Wash

Ag-RGO

OH OH

O COO -

Ag-PDDA

Ag

COO-

Graphene Oxide PDDA

+

N

OH + + + + NaBH4 + O + OH + OH + OH + hydrothermal + HO + + + O reaction + COO Ag+-PDDA-GO

Ag(NH3)2+ n

h

HO

+ Ag(NH3)2+ PDDA

OH

OH

as

O

W

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Figure 3. The schematic preparation mechanism of Ag-RGO in the absence and Ag-PDDA-RGO

presence of PDDA.

The crystallographic structures of the samples were obtained by XRD measurements. Figure 4 shows the XRD patterns of the natural graphite, GO and Ag-PDDA-RGO samples. It is known that the nature graphite has a strong diffraction peak at 26.5°, which corresponds to the (002) crystal plane. According to the Bragg equation, the layer-to-layer distance (d-spacing) is 3.4 Å.13 When the nature graphite 12


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is oxidized into GO, a new diffraction peak appears at 10.5°, corresponding to the (001) reflection. The increase of interlayer distance from 3.4 Å to 8.4 Å30 indicates the introduction of oxygen-containing functional groups. As for Ag-PDDA-RGO nanocomposites, the well-defined diffraction peaks at 38.1°, 44.3°, 64.4°, 77.4° and 82° are indicative of crystallographic planes of the cubic Ag crystal (JCPDS No. 04-0783).31 In addition, the relatively broad diffraction peaks of Ag indicate that the crystal size is very small. The average size of Ag nanoparticles is approximate 6.0 nm estimated from the parameters of XRD in accordance with the Scherrer formula. This result is consistent with that obtained from HRTEM image (Figure 2a). Moreover, disappearing of the diffraction peak (001) at 10.5° and forming of the (002) reflection at 24.5° demonstrate the efficient removal of oxygen-containing functional groups in Ag-PDDA-RGO hybrid.

Figure 4. XRD patterns of (a) Graphite, (b) GO and (c) Ag-PDDA-RGO.

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XPS analysis was carried out to investigate the elemental compositions and silver bonding configurations in these synthesized materials. The survey spectrum analysis in Figure 5a indicates the presence of C, O and Ag, which are confirmed by three peaks at 284.9, 532.9 and 368.7 eV, respectively.32 At the same time, as shown in Figure 5b, the spectrum of 3d core level of Ag consists of doublet peaks at 368.3 and 374.4 eV, corresponding to Ag 3d5/2 and Ag 3d3/2, respectively.33 There are about 6.0 eV spin-orbit splitting of the 3d doublet and no any additional peak shoulder, indicating the elementary substance nature of Ag(0) on RGO nanosheets.34 Figure 5c and 5d are the C 1s deconvoluted spectra of Ag-PDDA-RGO and GO, respectively. For Ag-PDDA-RGO, it can be seen that the peak intensity of two oxygen-containing functional groups (C-O and O-C=O) are weaken severely and the others (C-OH and C=O) evenly vanish with respect to GO. This also further proves that the atomic structure of RGO has been restored.35, 36 The UV-visible absorption spectra of GO, PDDA-RGO and Ag-PDDA-RGO are shown in Figure 6. It is shown that there are two absorption bands in the UV-visible spectrum of GO. The characteristic peak of GO centered at 229 nm and a shoulder peak at 303 nm are due to the π-π* transitions of aromatic C-C bonds and n-π* transitions of C=O bonds, respectively.37,38 After GO is reduced into RGO, the absorption peak of GO at 229 nm red-shifts to 274.5 nm and the shoulder peak of 303 nm disappears, indicating the partial restoration of electronic conjugation in the aromatic carbon structure of RGO. As for Ag-PDDA-RGO, besides of the characteristic peak of RGO at 274.1 nm, a new peak also appears at 436 nm, assigned

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to the surface plasmon resonance (SPR) absorption band of Ag nanoparticles, which confirms the formation of Ag nanoparticles.39, 40 These results are well consistent with those of the XRD and XPS analysis.

(a

(

(c

(

Figure 5. (a) XPS survey spectrum of Ag-PDDA-RGO sample, (b) the deconvoluted Ag 3d spectra of Ag-PDDA-RGO sample, and the deconvoluted XPS C1s spectra of (c) Ag-PDDA-RGO and (d) GO samples.

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Figure 6. UV spectra of (a) GO, (b) PDDA-RGO and (c) Ag-PDDA-RGO. To characterize the electrocatalytic activity of the as-prepared catalysts toward ORR, we first carried out the CV tests of Ag-PDDA-RGO in an Ar and O2-saturated 0.1M KOH solution, respectively, at a scan rate of 10 mV s-1. The CV curves are shown in Figure 7a. In the potential range from 0.35 to 1.55 V, there are two anodic peaks located at 1.29 V and 1.38 V, which are associated with the formation of bulk AgOH and Ag2O, respectively.41,42 During the negative scan, a significant cathodic peak appears at 1.01 V, suggesting the reduction of the silver oxides into metallic silver.43 When the electrolyte is saturated with O2, a well-defined reduction peak that emerges at about 0.822 V can be clearly seen, which is different from the indistinctive CV curve in Ar-saturated solution, indicating the occurrence of oxygen reduction reaction (inset of Figure 7a). To explicitly validate the ORR performance of Ag-PDDA-RGO, LSV tests were performed at various rotation speeds with a scan rate of 10mV s-1. It can be seen in Figure 7b that the ORR of Ag-PDDA-RGO is diffusion controlled at the potentials lower than 0.55 V, mixed diffusion kinetic controlled at the potentials from 0.55 V to 0.70 V and kinetic controlled at the potentials higher than 0.70 V. Figure 7b also indicates that high rotation speeds result in improving oxygen diffusion to the electrode surface and increasing diffusion current. Figure 7c presents the LSVs of different catalysts at the rotating speed of 1600 rpm.

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In general, the more positive the onset potential and the half-wave potential, the higher the activity of ORR catalyst. Obviously, the catalytic performance of PDDA-RGO is the worst. As for Ag-PDDA-RGO, the onset potential and half-wave potential are 0.93 V and 0.77 V, respectively, more positive than that of Ag-RGO (0.85 V, 0.73 V), indicating excellent electrocatalytic performance for ORR with respect to Ag-RGO. Though the half-wave potential of Ag-PDDA-RGO is slightly lower than that of Pt/C (0.80 V), the onset potential is comparable with that of Pt/C (0.93 V).44 The enhanced catalytic activity for ORR can also be proved by the relatively flat and wide current plateau similar to that of commercial Pt/C. The reason is because RGO has good electron transfer ability and there are abundant active sites for ORR on the surface of Ag-PDDA-RGO due to no agglomeration of Ag nanoparticles and RGO with the existence of PDDA. Figure 7d shows the Koutecky-Levich (K-L) plots which describe the relation between the inverse real current density (j-1) and the inverse of square root of the rotating rate (ω-1/2).45 The data of j and ω are taken from the diffusion-controlled region in Figure 7b. It can be seen from Figure 7d that the corresponding K-L plots exhibit good linearity and the slopes remain almost unchanged over the potentials range from 0.2 V to 0.5 V, indicating the electron transfer numbers for ORR are similar at different potentials. The electron transfer number (n) can be calculated from the slope of K-L plots by K-L equation, which is given as below: 46 1 1 = + j jk

1 1

0 .62 nFD

2/3 0

ν −1 / 6 ω 2 C 0 17



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Where j is the measured current density; jk is the kinetic current density; n is the transferred electron number per oxygen molecule involved in the ORR; F is the Faraday constant (F = 96485 C mol-1); D0 is the diffusion coefficient of O2 in 0.1M KOH (D0 = 1.9×10-5 cm2 s-1); υ is the kinematic viscosity of 0.1M KOH (υ = 1.0×10-2 cm2 s-1); C0 is the bulk concentration of O2 in the electrolyte (C0 = 1.2×10-3 mol L-1), and ω is the rotating rate of the electrode. The inset of Figure 7d shows the average number(aof electrons transferred is 3.94 in(bthe potential range from 0.2 V to 0.5 V, suggesting that the ORR process catalyzed by the Ag-PDDA-RGO takes place through the most efficient four-electron way.47

(c

(d

Figure 7. (a) CV curves of Ag-PDDA-RGO sample in Ar-saturated (black line) and O2-saturated (red line) 0.1 mol L−1 KOH solution with a sweep rate of 10 mV s-1. (b) LSV curves of Ag-PDDA-RGO sample in O2-saturated 0.1 mol L−1 KOH solution at

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various rotation speeds with a sweep rate of 10 mV s-1. (c) ORR polarization curves for PDDA-RGO, Ag-RGO, Ag-PDDA-RGO and Pt/C electrocatalysts in O2-saturated 0.1 mol L−1 KOH solution at a rotation speed of 1600 rpm with a sweep rate of 10 mV s-1. (d) The Koutecky–Levich plots of Ag-PDDA-RGO at different potentials, inset image of electron transfer number at corresponding potential.

To research the effect of energy-saving electrolysis of sodium carbonate, the synthesized Ag-PDDA-RGO was prepared as catalyst layer of the oxygen reduction cathode (ORC) to replace the conventional hydrogen evolution cathode (HEC). Figure 8 presents the principle schematic for energy-saving electrolysis of Na2CO3 by ORC. As shown in Figure 8, the concentrated NaOH and NaHCO3 are generated in the cathodic and andodic chambers, respectively. The concentration of obtained NaOH and NaHCO3 after electrolysis are 6.61 M and 1.01 M, which are suitable for digestion of bauxite ores and precipitation of Al(OH)3, respectively.

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Figure 8. The schematic of energy-saving electrolysis of Na2CO3 with ORC.

The galvanostatic electrolysis was carried out at 100 mA cm-2 for 4h to compare the energy consumption of HEC and ORC with different catalysts. Figure 9a shows that the average cell voltage using ORC with PDDA-RGO, Ag-RGO and Ag-PDDA-RGO is 1.72 V, 1.61 V and 1.48 V, respectively. That indicates Ag-PDDA-RGO has the highest catalytic activity for ORR, which is consistent with the results in Figure 7c. It can also be seen from Figure 9a that the average cell voltage using HEC is about 2.53 V, which basically coincides with that in our previous study.6,7 The average cell voltage using ORC with Ag-PDDA-RGO, is 1.05 V lower than that using HEC. Thus corresponding up to 41.5% of electrical energy is saved when the HEC is replaced by the ORC with Ag-PDDA-RGO at the same current density of 100 mA cm-2.

Figure 9. (a) the average cell voltages of Na2CO3 electrolysis using conventional HEC and ORC prepared with PDDA-RGO, Ag-RGO and Ag-PDDA-RGO at 100 mA cm-2, and (b) the stability of Ag-PDDA-RGO as ORC in energy-saving electrolysis of Na2CO3.

The stability of catalyst is a very important factor to determine whether as-prepared 20


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catalysts can be industrialized. Figure 9b also shows the variation of cell voltage with electrolysis time. The electrolysis with ORC was repeated three times. As seen from Figure 9b, the cell voltage keeps almost unchanged with increasing times of electrolysis, indicating that the ORC prepared with Ag-PDDA-RGO catalyst has high stability during electrolysis. It can also be seen that the cell voltage slightly increases with prolonging the electrolysis time, which might be due to the change in electrolyte pH. In conclusion, the ORR catalyst of Ag-PDDA-RGO is promising for the development of energy-saving electrolysis of Na2CO3.

4. CONCLUSIONS In this paper, the highly dispersed Ag-functionalized graphene nanocomposites were synthesized by one-step facile hydrothermal method with the presence of PDDA. The morphology of the electrocatalysts was characterized by XRD, XPS, TEM and HRTEM. The results show that Ag with particle size of about 5.7 nm homogeneously distributes on the surface of sheet-like RGO. Meanwhile, the electrochemical test shows Ag-PDDA-RGO nanocomposite has excellent electrocatalytic activity towards ORR. The constant-current electrolysis demonstrates that the cell voltage is decreased by 1.05 V and correspondingly the energy consumption is reduced by 41.5% when ORC replaces the conventional HEC. The ORC prepared with Ag-PDDA-RGO also exhibits super good stability during the electrolysis. Thus, the aim of energy-saving electrolysis of NaCO3 can be realized and the as-prepared is promising to be applied in the alumina industry.

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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS

This work was supported by National Natural Science Foundation of China (No. 51374016 & 21476022), the State Key Program of National Natural Science of China (21236003), the Fundamental Research Funds for the Central Universities (No. JD1515 & JD1415) and Beijing Higher Education Young Elite Teacher Project (No. YETP0509).

SUPPORTING INFORMATION AVAILABLE

The supporting information (SI) document contains: S1) The FTIR spectra of pure PDDA (black line) and Ag-PDDA-RGO (red line); S2) The TEM images of Ag-PDDA-RGO (a) with and (b) without addition of ammonia. S3) references sited in the SI. This information is available free of charge via the Internet at http://pubs.acs.org/.

REFERENCES

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Highly Dispersed Ag-Functionalized Graphene Electrocatalyst for

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