Preparation and Electrocatalytic Property of Triuranium Octoxide Supported on Reduced Graphene Oxides

Nano Research 1 DOINano 10.1007/s12274Res -014-0668-8

Dongliang Gao1, 2, Zhenyu Zhang1, Li Ding1, Juan Yang1, and Yan Li1, 2 ()

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0668-8 http://www.thenanoresearch.com on December 2 2014

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Preparation and Electrocatalytic Property of Triuranium Octoxide Supported on Reduced Graphene Oxides

Dongliang Gao1, 2, Zhenyu Zhang1, Li Ding1, Juan Yang1, and Yan Li1, 2*

1 Key Laboratory for the Physics and Chemistry of Nanodevices, Beijing National Laboratory for Molecular Science, College of Chemistry and Molecular Engineering, State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing100871, China A two-step solution-phase method was used to prepare triuranium octoxides-reduced graphene oxides hybrids, which exhibited superior electrocatalytic activity for oxygen 2 Academy for Advanced Interdisciplinary reduction reaction. Studies, Peking University, Beijing100871, China

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Nano Research

DOI (automatically inserted by the publisher) Research Article

Preparation and Electrocatalytic Property of Triuranium Octoxide Supported on Reduced Graphene Oxides

Dongliang Gao1, 2, Zhenyu Zhang1, Li Ding1, Juan Yang1, and Yan Li1, 2 ()

Received: day month year ABSTRACT

Revised: day month year Triuranium octoxides-reduced graphene oxides (U3O8/rGO) hybrids were Accepted: day month year prepared by a two-step solution-phase method. The presence of GO is essential (automatically inserted by for obtaining pure phase U3O8. The U3O8/rGO hybrids exhibited superior the publisher) electrocatalytic activity for oxygen reduction reaction. The electron transfer number was calculated to be ~3.9 at -0.7 V (v.s. Ag/AgCl) from the slope of © Tsinghua University Press Koutecky-Levich plots. The U3O8/rGO hybrids were more stable than the and Springer-Verlag Berlin commercial Pt/C catalysts. When methanol existed, the U3O8/rGO hybrids still Heidelberg 2014 kept high activity. Besides, the U3O8/rGO hybrids can also catalyze the reduction of hydrogen peroxide. KEYWORDS Triuranium Octoxide, Reduced Graphene Oxides, Oxygen Reduction Reaction,

Electrocatalysis

of 99.275%, 0.720%, and 0.005%, respectively [1]. 1 Introduction Uranium-dioxide with enriched 235U is normally used as a fuel in the nuclear reactors. So large amount of 238U, which has a very long half-life period of ~4.5 Uranium is an important element in nuclear industry. billion years and hence is safe to be used Uranium consists of several natural isotopes conventionally [2], is left. Therefore, the application including 238U, 235U, and 234U, with natural abundance

Address correspondence to Yan Li, [email protected]

2 Nano Res. of residual 238U is of great importance. [19]. In these studies, it is always difficult to avoid the formation of impurity uranium oxides with Uranium is an actinide element which has 5f different valence. The introducing of different electrons. The 5f-orbital can hybridize with the organic molecules which act as reductants or capping 6d-orbital, giving the actinides a broader range of agents is also a big problem, which depresses the oxidation states. Thus uranium possesses +2, +3, +4, catalytic performance of uranium oxides [20]. +5, and +6 valence [3-9]. Besides the most outside orbitals in the 7th shell, both 5f and 6d orbitals can Graphene oxide (GO) presents high specific also partake in chemical bonding, therefore, the surface area and contains carboxylic, hydroxyl, bonding of uranium is quite complicated. In addition, epoxide and other hydrophilic functional groups on chemical bonds consisting of uranium ions are often the surface. Therefore, GO has been widely used as less ionic due to the large radius and high charge substrates to prepare inorganics-GO hybrids. Dai’s number [10]. So uranium has different coordination group has developed a general two-step method to numbers and bonding modes from the lanthanide or prepare hybrids of inorganic nanomaterials and transition elements [11]. Owing to the special graphene oxides. First, metal ions absorb onto GO structural features and chemical properties, uranium and hydrolyze in situ; then the pre-products are oxides may be used as good catalysts for different treated under hydrothermal or solvothermal kinds of reactions [10, 12-16]. conditions to obtain the final hybrids [22-30]. This method may be used to prepare hybrids of uranium It is found that the oxidation state of uranium is a oxides and GO. crucial factor influencing its catalytic performance. For instance, U3O8 can catalyze the aldolization With the outstanding properties of high electrical reaction of acetaldehyde to form crotonaldehyde. conductivity, surface area, flexibility, thermal

However, when β-UO3 was used as catalysts, conductivity, and mechanical strength, graphene can acetaldehyde conducted condensation reaction to be used as electrode materials or supports for form furan [17]. U3O8 nanocrystals have a better electrocatalysts [31-33]. Due to the scarcity and high catalytic performance for benzyl alcohol conversion price of Pt for large scale application of fuel cells, to benzadlehyde than UO2 nanocrystals [18]. electrocatalysts without Pt have attracted much Nanoplates of uranium oxide hydroxide hydrate attention [34-36]. Very recently, M. Pumera et al. exhibit a higher catalytic activity than U3O8 for reported that uranium doped graphene hybrids benzyl alcohol oxidation [15]. The oxygen-defected exhibited electrocatalytic properties towards oxygen

UO2 (111) single crystal reduces coupling of CO reduction [37]. They found UO3 and U3O8 co-exist in molecules to acetylene and ethylene compounds on their catalysts. It is unclear which component acts as its surface [12]. the catalytic species and the performance of the catalysts is not optimized. In this paper, we prepared Thus synthesis of pure phase uranium oxides with hybrids of pure phase triuranium octoxide and different oxidation states is very important for reduced GO (U3O8/rGO) and studied their studying their catalytic property. A few literatures electrocatalytic property. It was found that the have reported about the preparation of uranium U3O8/rGO hybrids showed very good activity toward oxides nanomaterial [15, 18-21]. For instance, using oxygen reduction reaction (ORR) and were more different organic amines as reducing reagents, UO2 stable than the commercial Pt/C catalysts. U3O8/rGO nanospheres and U3O8 nanorods have been hybrid catalysts might be used as a substitute of Pt/C synthesized [18]. By the addition of hydrazine, electrocatalyst for oxygen reduction reaction in fuel spherical UO2 nanoparticles with diameter from 30 to cells.

250 nm and U3O8 nanocuboids have been synthesized

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voltammetry. The catalyst-modified electrode was 2 Experimental prepared by transferring 20 μl of 2 mg/ml suspension of the samples onto the glassy carbon

2.1 Preparation of U3O8/rGO nanocrystals electrode with a diameter of 5.0 mm. A thin layer of Nafion was added to cover the electrode when it GO was synthesized by an improved hummers was dried. ORR was carried out in O2-saturated 0.1 method [38] (see details in the supporting M KOH aqueous solution at room temperature. information). Typically, 6 mg GO was dispersed in Hydrogen peroxide reduction was carried in 10 ml ethanol. 1 ml of 30 mg/mL UO2(NO3)2 N2-saturated 50 mM phosphate buffer saline (PBS) aqueous solution and 0.9 mL of concentrated aqueous solution at room temperature. NH3·H2O was injected into the suspension at 60 ºC, respectively. After stirring at 60 ºC for 6 hours, the 3 Results and discussion precipitates were collected by centrifugation and washed with water. Then the precipitates were Figures 1 (a) & (c) shows the TEM and SEM images re-dispersed in 10 ml water and transferred into a of the products obtained under typical conditions 20 ml Teflon-lined stainless steel autoclaves to carry with the hydrolysis temperature of 60 ºC. HRTEM out hydrothermal reaction at 200 ºC for 4 hours. The image in Fig. 1(b) clearly exhibits the fringes with product was collected by centrifugation, washed the inter-plane distance of (0 0 1) plane of U3O8 (0.42 with water, frozen by liquid nitrogen and nm). The selected area electron diffraction (SAED) lyophilized overnight. Using lyophilization process pattern also matches the structure of single crystal other than normal drying can avoid the aggregation U3O8 well. So the product we obtained is U3O8/rGO. of the products. The morphologies of the U3O8 nanoparticles are approximately cuboid with the dimensions of 2.2 Characterization 100-200 nm in length and 20-100 nm in width. X-ray diffraction (XRD) measurements were performed on a Rigaku Dmax-2400 diffractometer using Cu-Kα radiation (λ = 1.5406 Å ) with an accelerating voltage of 40kV. Particle shape and morphology of the products were observed with a scanning electron microscope (SEM, Hitachi S-4800) and a transmission electron microscope (TEM, FEI Tecnai G2 T20).

2.3 Electrochemical measurements

Electrochemical experiments were carried out on an Figure 1 TEM (a), HRTEM (b), and SEM (c) images and the electrochemical workstation (CHI 760e) by cyclic XRD pattern (d) of U3O8/rGO hybrids. The insert in (b) is the and linear scanning voltammetry technique. SAED pattern of U3O8. Ag/AgCl/KCl (saturated) electrode was used as As shown in Fig. S2, the products after hydrolysis reference electrode and Pt wire was used as counter were proved to be uranium ammine oxide hydrate electrode. Rotating disk electrode (Pine Research (2UO3·NH3·3H2O). U(VI) must be reduced during Instrument) with glassy carbon (GC) substrate was the hydrothermal treatment. There are some used as working electrode for the linear scanning amount of very tiny carbonaceous fragments in the

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4 Nano Res. dispersion of GO, which might be able to act as hydrothermal treatment. reductants. NH3 also presents reducibility. More importantly, all peaks in the XRD pattern of Therefore, we guess that the tiny carbonaceous the product can be indexed to orthorhombic U3O8 fragments and NH3H2O may act together as the phase (PDF#2-276), and no peaks of impurities are reductants for both U(VI) and GO during observed (Fig. 1(d)). This indicates we have hydrothermal treatment. 2UO3·NH3·3H2O is obtained pure phase U3O8. However, we found that transformed to U3O8 and GO becomes rGO. U3O8 could not form in the absence of GO. Instead, uranium oxides sheets with the size of 500 nm to 1000 nm were formed (Fig. S1(a)). The XRD patterns shown in Fig. S1(b) proved that the product is a mixture. It seems that GO provides a suitable

environment for the formation U3O8.

Figure 3 CV curves of U3O8/rGO hybrids on GC electrodes in

O2-saturated and N2-staturated 0.1 M KOH at a scan rate of 50 mV·s-1. Besides the presence of GO, hydrolysis temperature and solvents used in solvothermal

treatment are also crucial for the synthesis of pure U3O8. A lower hydrolysis temperature is important Figure 2 CV (a) and LSV (b) curves of oxygen reduction on to slow down the reaction rate and avoid the rGO, UOx, U3O8/rGO, and Pt/C modified GC electrodes and formation of 2UO3·NH3·3H2O in solution other than bare GC electrode in O2-saturated 0.1 M KOH at a scan rate of on GO. It is found that at 60 C, particles were only 50 mV·s-1 (CV) and 10 mV·s-1 (LSV), respectively. formed on GO. This is important for the Figure S3 & S4 show the XRD patterns of preparation of pure phase U3O8. But at 80 C, some products after 1h and 2h hydrothermal treatment, isolated particles were formed in solution, as a respectively. From the XRD patterns, we could see result, pure phase U3O8 were not obtained (Fig S5). that U3O8 and 2UO3·NH3·3H2O particles were both When ethanol was used instead of water in existence, indicating that 2UO3·NH3·3H2O particles solvothermal treatment, we also failed to prepare were partially reduced after 1h and 2h pure phase U3O8 (Fig. S6).

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Then the electrocatalytic property of U3O8/rGO We used linear sweep voltammetry (LSV) on was investigated. First, ORR was studied with cyclic rotating-disk electrode (RDE) measurements to voltammetry (CV) measurements in O2-saturated further investigate the performance of the

0.1 M KOH aqueous solutions. The CV curves of U3O8/rGO hybrids catalyzing ORR. Figure 4(a) is bare GC and modified electrodes with commercial the rotating-disk voltammograms at different

Pt/C catalysts, rGO, UOx, and U3O8/rGO loaded rotation rates. The limiting current density were shown in Fig. 2(a). Bare GC, rGO, and UOx all increased with the increasing of rotation rate. The exhibited very poor catalytic activity. The U3O8/rGO ORR kinetics of the electrodes was analyzed by the hybrids showed a much more positive ORR onset Koutecky-Levich equation [Eqs. (1), (2), and (3)]: potential and higher cathodic current. The (1) U3O8/rGO hybrid had an ORR onset potential at -0.18V v.s. Ag/AgCl according to the LSV (Fig. 2(b)). (2) The CV of U3O8/rGO hybrids showed no peak in the

N2-saturated 0.1M KOH aqueous solutions, which (3) proved the peak we observed is not the reduction in which j is the measured current density, jk is the peak of the U3O8 nanocrystals themselves (Fig. 3). kinetic limiting current density and j1 is the diffusion limiting current density. The variable ω represents the rotation rate of the electrode [rpm], n

is the number of electrons transferred per O2 molecule, F is the Faraday constant (96485 C·mol-1),

CO is the bulk concentration of O2 in 0.1 M KOH (1.9

10-5 cm2·s-1), DO is the diffusion coefficient of O2 in 0.1 M KOH (1.910-5 cm2·s-1) v is the kinetic viscosity of the electrolyte (0.01 cm2·s-1) and k is the electron-transfer rate constant [39].

The transferred electron number (n) and the kinetic

limiting current density (jk) of the electrodes can be determined from the slope and intercept of the Koutecky-Levich plots, respectively. Figure 4(b) demonstrates the Koutecky-Levich plots of the

U3O8/rGO hybrids at various potentials. Both of them had linear relationship between j-1 and ω-0.5. n was calculated to be ~3.9 at -0.7 V from the slope of

Koutecky-Levich plots, suggesting U3O8/rGO hybrids favors a 4 electron oxygen reduction process, similar to the ORR catalyzed by a high-quality commercial Pt/C catalyst measured in the same 0.1 M KOH

electrolyte. Figure 4 (a) LSV curves for U3O8/rGO modified electrode at various rotation in O2-saturate 0.1 M KOH at a scan rate of 10 The stability of the U3O8/rGO hybrids catalyst -1 mV·s . (b) Koutecky-Levich plots of the U3O8/rGO modified was evaluated by chronoamperometry in electrode at different potentials. O2-saturated 0.1 M KOH aqueous solutions (Fig

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6 Nano Res.

5(a)). The relative current for U3O8/rGO hybrids observed for the U3O8/rGO electrode after the decreased about 8% after 20000 s, while the relative addition of methanol (~1.0 ml), while the current current for Pt/C decreased 14% under the same decrease sharply for the Pt/C electrode. This conditions. We performed XRD measurements for indicates U3O8/rGO hybrids have a good resistance the hybrids after chronoamperometry of 20000 s to crossover effects when used in fuel cells. and found U3O8 maintained its structure (Fig. S7). From the above results, our U3O8/rGO hybrids

This result indicates the U3O8/rGO hybrids are more showed superior catalytic performance for ORR stable than the commercial Pt/C catalyst. Actually, reaction compared with U-doped graphene, where our catalysts show quite satisfying stability among UO3 and U3O8 coexist [37]. In U3O8, uranium the reported non-Pt catalysts [36]. possesses the valence of +4 and +6. We also found

that 2UO3·NH3·3H2O/rGO, in which uranium possesses the valence of +6, afforded lower ORR activities (Fig. S8). It has been reported that electrocatalytic ORR in alkaline media proceeds through multistep reactions, first,

HO2- intermediates are formed from the adsorbed

O2 on the active sites of the catalyst surface, then further reduced or decomposed to OH- ions [40]. So we propose that U(IV) may act as the active site for the reduction of oxygen species and the transformation between U(IV) and U(VI) has a significant effect on the catalytic activity.

Figure 5 Chronoamoerometric responses (percentage of current retained versus operation time) of Pt/C and U3O8/rGO modified

GC electrode at -0.3 V (a) and with 3 M methanol added at

~1400 s (b) in O2-saturate 0.1 M KOH at a rotation rate of 1000 Figure 6 CV curves of hydrogen peroxide reduction on the rpm. U3O8/rGO modified GC electrode in 50 mM PBS at a scan rate of 50 mV·s-1. Figure 5(b) showed the chronoamperometric Electrochemical measurements were also responses of the Pt/C and U3O8/rGO modified conducted for hydrogen peroxide reduction in 50 electrodes in O2-saturated 0.1 M KOH aqueous mM PBS aqueous solution. A reduction peak was solutions before and after the addition of methanol. found for the U3O8/rGO hybrids at -0.83 V v.s. A little decrease of the reaction current was

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Ag/AgCl (Fig. 6). So the U3O8/rGO hybrids also Uranium(III) and Uranium(IV) - Synthesis of Delta-Hydrocarbyl Derivatives of Uranium(IV) and present electrocatalytic activity toward the Reactivity of UCl2R[HB(3,5-Me2pz)3] (R = CH2SiMe3, reduction of hydrogen peroxide. CH(SiMe3)2) and UCl2[HB(3,5-Me2pz)3] toward Ketones and Aldehydes. Organometallics 1994, 13, 654-662. [6] Weydert, M.; Brennan, J. G.; Andersen, R. A.; Bergman, R. 4 Conclusion G. Reactions of a Uranium(IV) Tertiary Alkyl Bond - Facile Ligand-Assisted Reduction and Insertion of Ethylene and By first hydrolysis in ethanol solution and then Carbon-Monoxide. Organometallics 1995, 14, 3942-3951. hydrothermal treatment, U3O8/rGO hybrids were [7] Privalov, T.; Schimmelpfennig, B.; Wahlgren, U.; Grenthe, prepared. The presence of GO offered a suitable I. Structure and thermodynamics of uranium(VI) complexes in the gas phase: A comparison of experimental condition for the formation of pure phase U3O8. and ab initio data. J. Phys. Chem. A 2002, 106, Such a pure phase U3O8 exhibited superior 11277-11282. electrocatalytic activity. For catalyzing oxygen [8] Duttera, M. R.; Fagan, P. J.; Marks, T. J.; Day, V. W. reduction reaction, U3O8/rGO hybrids performed a Synthesis, Properties, and Molecular-Structure of a favorable 4 electron process and showed higher Trivalent Organouranium Diphosphine Hydride. J. Am. stability than the commercial Pt/C catalysts. Unlike Chem. Soc. 1982, 104, 865-867. the immediate deactivation of Pt/C, U3O8/rGO [9] Sturchio, N. C. Tetravalent Uranium in Calcite. Science hybrids electrocatalysts maintained their high 1998, 281, 971-973. activity with the existence of methanol. Besides, [10] Idriss, H. Surface reactions of uranium oxide powder, thin films and single crystals. Surf. Sci. Rep. 2010, 65, 67-109. U3O8/rGO hybrids can also catalyze the reduction of [11] Fox, A. R.; Bart, S. C.; Meyer, K.; Cummins, C. C. hydrogen peroxide. It is expected that uranium Towards uranium catalysts. Nature 2008, 455, 341-349. oxides may find more important applications in [12] Senanayake, S. D.; Waterhouse, G. I. N.; Idriss, H. electrocatalysis. Coupling of carbon monoxide molecules over oxygen-defected UO2(111) single crystal and thin film Acknowledgements surfaces. Langmuir 2005, 21, 11141-11145. [13] Amrute, A. P.; Krumeich, F.; Mondelli, C.; Pérez-Ramírez, This work is supported by Ministry of Science and J. Depleted uranium catalysts for chlorine production. Technology of China (Project 2011CB933003) and the Chem. Sci. 2013, 4, 2209-2217. National Natural Science Foundation of China [14] Amrute, A. P.; Krumeich, F.; Mondelli, C.; Pérez-Ramírez, (Projects 11179011, 21125103, and 91333105). J. Depleted uranium catalysts for chlorine production. Chem. Sci. 2013, 4, 2209. [15] Pradhan, M.; Sarkar, S.; Sinha, A. K.; Basu, M.; Pal, T. References Morphology controlled uranium oxide hydroxide hydrate for catalysis, luminescence and SERS studies. [1] Nier, A. The Isotopic Constitution of Uranium and the CrystEngComm 2011, 13, 2878. Half-Lives of the Uranium Isotopes. I. Phys. Rev. 1939, 55, [16] Hutchings, G. J.; Heneghan, C. S.; Hudson, I. D.; Taylor, S. 150-153. H. Uranium-oxide-based catalysts for the destruction of [2] Audi, G.; Bersillon, O.; Blachot, J.; Wapstra, A. H. The volatile chloro-organic compounds. Nature 1996, 384, Nubase evaluation of nuclear and decay properties. Nucl. 341-343. Phys. A 2003, 729, 3-128. [17] Madhavaram, H.; Idriss, H. Acetaldehyde reactions over [3] Gresham, G. L.; Gianotto, A. K.; Harrington, P. D.; Cao, L. the uranium oxide system. J. Catal. 2004, 224, 358-369. B.; Scott, J. R.; Olson, J. E.; Appelhans, A. D.; Van [18] Wang, Q.; Li, G.-D.; Xu, S.; Li, J.-X.; Chen, J.-S. Synthesis Stipdonk, M. J.; Groenewold, G. S. Gas-phase hydration of of uranium oxide nanoparticles and their catalytic U(IV), U(V), and U(VI) dioxo monocations. J. Phys. Chem. performance for benzyl alcohol conversion to A 2003, 107, 8530-8538. benzaldehyde. J. Mater. Chem. 2008, 18, 1146-1152. [4] Hargreaves, W. High-Resolution Measurements of [19] Zhao, R.; Wang, L.; Gu, Z.-J.; Yuan, L.-Y.; Xiao, C.-L.; Absorption, Fluorescence, and Crystal-Field Splittings of Zhao, Y.-L.; Chai, Z.-F.; Shi, W.-Q. A facile additive-free Solutions of Divalent, Trivalent, and Tetravalent Uranium method for tunable fabrication of UO2 and U3O8 Ions in Fluoride Crystals. Phys. Rev. 1967, 156, 331-342. nanoparticles in aqueous solution. CrystEngComm 2014, [5] Domingos, A.; Marques, N.; Dematos, A. P.; Santos, I.; 16, 2645. Silva, M. Hydrotris(Pyrazolyl)Borate Chemistry of [20] Bouala, G. I. N.; Clavier, N.; Podor, R.; Cambedouzou, J.;

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Electronic Supplementary Material

Preparation and Electrocatalytic Property of Triuranium Octoxide supported on Reduced Graphene Oxides

Dongliang Gao1, 2, Zhenyu Zhang1, Li Ding1, Juan Yang1, and Yan Li1, 2 ()

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

1. Experimental

1.1 Materials

Kish Graphite was purchased from Aldrich Reagent. High purity 238U3O8 was obtained from China National Nuclear Corporation. Perfluorosulfonic acid-PTFE copolymer, 5% w/w solution, was purchased from Alfa Aesar. All other reagents used are of A.R. grade.

1.2 Preparation of GO

Graphite oxide was synthesized by an improved Hummers method (Ref. [34] in main text). A 9:1 mixture of

H2SO4/H3PO4 (135:15 ml) was added to a mixture of graphite flakes (1.0 g) and KMnO4 (6.0 g), then heated to 50 ℃ and stirred for 24 h. Afterwards, the reaction mixture was cooled to room temperature and poured onto ice (from 600 ml water) with 30% H2O2 (9 mL). Then, the mixture was centrifuged at 10000 rpm for 10 min. The remaining solid material was washed in succession with deionized water for six times. For each round, the mixture was centrifuged at 10000 rpm for 20 min to obtain the precipitates. The final precipitates were frozen by liquid nitrogen and lyophilized overnight.

Address correspondence to Yan Li, [email protected]

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1.3 Preparation of uranyl nitrate

Concentrated nitric acid (2 ml) was added to U3O8 (2.5 g), and eventually a yellow solution was obtained. The raw products was crystalized out from the solution by heating and were further purified by three times recrystallization. Finally, lemon-yellow-colored uranyl nitrate crystals were obtained.

2. Supplementary figures

Figure S1 (a) TEM image and (b) XRD pattern of the UOx nanoparticles obtained without the presence of GO.

Figure S2 (a) TEM image and (b) XRD pattern of hydrolysis production.

Figure S3 XRD pattern of products after 1 h hydrothermal. Figure S4 XRD pattern of products after 2 h hydrothermal Treatment treatment.

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Figure S5 (a) TEM image and (b) XRD pattern of UOx/rGO obtained at the hydrolization temperature of 80 ºC other than 60 ºC.

Figure S6 (a) TEM image and (b) XRD pattern of UOx/rGO obtained by solvothermal treatment in ethanol instead of water.

Figure S7 XRD pattern of U3O8/rGO hybrids after 20000s of chronoamperometry scanning.

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Figure S8 LSV curves of oxygen reduction on 2UO2·NH3·3H2O modified GC electrodes in O2-saturated 0.1 M KOH at a scan rate of 10 mV·s-1.

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