- Email: [email protected]
RuO2 supported on Sb-doped SnO2 nanoparticles for polymer electrolyte membrane water electrolysers
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 5 8 0 6 e5 8 1 0
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
RuO2 supported on Sb-doped SnO2 nanoparticles for polymer electrolyte membrane water electrolysers Xu Wu, Keith Scott* School of Chemical Engineering and Advanced Materials, Merz Court, Newcastle University, Newcastle upon Tyne, NE17RU, UK
article info
abstract
Article history:
With a colloid method, RuO2 was deposited on Sb-doped SnO2 nanoparticles (ATO, Aldrich,
Received 29 September 2010
30e40 nm), which was employed as a novel support material for anode catalysts of poly-
Accepted 29 October 2010
mer electrolyte membrane water electrolysers (PEMWE). Distinctive RuO2 nanoparticles
Available online 3 April 2011
(10e15 nm) were stably deposited on ATO nanoparticles, which were characterized with XRD and SEM. RuO2/ATO exhibited higher activity than unsupported RuO2 for oxygen
Keywords:
evolution. A PEMWE single cell with 10 mg cm2 20 wt.% RuO2/ATO achieved 1.56 V at
Water electrolyser
1 A cm2 at 80 C.
Electrolysis
Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Oxygen evolution reaction
reserved.
PEMWE RuO2 Sb-doped SnO2
1.
Introduction
The anode catalyst has been the focus of many studies on polymer electrolyte membrane water electrolysers (PEMWE), because that the oxygen evolution reaction (OER) is widely considered the largest cause of overpotential. Typically only those materials based upon expensive RuO2 and IrO2 are regarded as suitable (stable and active) catalysts for OER in acidic medium [1e10]. One method to achieve better performances and lower noble metal use is to utilize catalyst supports, which may increase the dispersion and surface exposure of catalysts, i.e. a substantial increase in catalyst utilization [11]. Also support materials could increase electronic conductivity of OER catalysts, which are usually semiconductive oxides [11,12], and increase catalyst stability [13,14]. Typically, carbon has been used as catalyst support in applications such as polymer electrolyte membrane fuel cells.
However, the electrochemical oxidation of carbon during oxygen evolution in the PEMWE will limit the practical lifetime of catalyst supported on carbon. Materials such as TiC [11], TiO2 [12], SnO2 [13,14] etc. have been demonstrated as supports of oxygen evolution catalysts. It’s commonly found that RuO2 exhibits less overpotential than IrO2. Moreover, RuO2 is several times cheaper than IrO2. Therefore, RuO2 is the most widely employed OER catalyst of dimensionally stable anodes (DSA) in the chlor-alkali industry. Due to their unstable octavalent intermediates, RuO2 electrodes might have insufficient stability. However, mixed oxides such as RuO2eSnO2 have exhibited much more stable performances than RuO2 [14e16]. In this case, SnO2 particles function as both the support material and stabilizer. Since SnO2 is a semiconductor, it’s a reasonable view that properties of RuO2 supported by SnO2 can be improved by enhancing the electronic conductivity of the catalyst support. With this consideration, Sb
* Corresponding author. Tel.: þ44 (0)1912228771; fax: þ44 (0)1912225292. E-mail address: [email protected] (K. Scott). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.10.098
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 5 8 0 6 e5 8 1 0
(V) doped SnO2 (ATO) could be a good alternative, since it exhibits higher electronic conductivity than SnO2, attributing to pentavalent Sb doping. ATO nanoparticles have been recently reported as a promising support material for Pt [17] for fuel cells. More recently, RuO2eIrO2 supported on ATO was studied and reported as a potentially promising catalyst for water electrolysers [18]. However, there are no published performances of PEMWE using ATO nanoparticles as support of the anode catalysts. In this communication, RuO2 was deposited on ATO nanoparticles by a novel modified colloid method. The activity of RuO2/ATO in a PEMWE single cell was demonstrated.
2.
Experimental
2.1.
Catalyst preparation
Ruthenium chloride hydrate (RuCl3$H2O, Alfa Aesar) was dissolved in an aqueous solution (40e50 cm3) containing 1.05 103 mol NaOH, with stoichiometric ratio Ru:NaOH z 1:7. With stirring and N2 bubbling, the solution turned to light green after several hours. Consequently, 80 mg ATO powders (Aldrich, 10 mol% Sb, 30 nm) were added into the solution, so that the final weight ratio of RuO2:ATO z 1:4. Then, after being ultrasonicated for 5 min, the pH value of this solution was adjusted to about 8.0 by slowly titration of a 0.1 mol dm3 HClO4 solution. The solution eventually turned achromatous when a precipitate was obtained after four days. The following reactions happened in this preparation procedure: 3 RuCl3 þ 6OH / RuðOHÞ6 þ3Cl
(1)
3 RuðOHÞ6 þ3Hþ /RuðOHÞ3 ðH2 OÞ3 Y
(2)
7Cl þ ClO4 þ 8Hþ /4Cl2 [ þ 4H2 O
(3)
The hydroxyls in the alkaline solution would substitute chlorions of RuCl3$H2O and a [Ru(OH)6]3 ion would form due to sp3d2 hybridization of Ru3þ, as shown in chemical equation (1). When HClO4 was titrated into the solution, the precipitation of electroneutral Ru(OH)3(H2O)3 and volatilization of chlorine would be the dominant reactions, as equations (2) and (3). The size of Ru(OH)3(H2O)3 could be reduced by slowing the precipitation process. The resulting precursor of Ru(OH)3(H2O)3/ATO was separated with a centrifuge, washed with deionised water several times, and dried at 70 C overnight. Finally, the precursor was heated to 450 C in air with a heating rate of 3 C min1, and then annealed at 450 C for 2 h. During the heattreatment process, Ru(OH)3(H2O)3 would be oxidized to RuO2 and excess water would evaporate, as equation (4). 4RuðOHÞ3 ðH2 OÞ3 =ATO þ O2 / 4RuO2 =ATO þ 18H2 O[
(4)
Another sample RuO2 without ATO was prepared with the same procedure for comparing.
2.2.
Characterizations
The sample RuO2/ATO and ATO powders were analyzed with X-ray diffraction (XRD, PANalytical X’Pert Pro Diffractometer), scanning electron microscopy (SEM, JEOL JSM5300LV), and
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Energy dispersive X-ray spectroscopy (EDX, Rontec Quantax) attached to SEM. Phase identification was carried out by X’Pert High Score software and the ICDD database PDF2-2004.
2.3.
Electrochemical investigation
Electrochemical experiments were carried out in a 3-electrode cell in a 0.5 mol dm3 H2SO4 solution prepared with deionised water. A saturated calomel electrode reference electrode (SCE) was connected to a Luggin capillary which was positioned close to the working electrode. The counter electrode was a Pt foil (1 cm 2.5 cm). For preparing working electrodes, the RuO2/ATO (of which RuO2 is approximately 20% of total weight) and Nafion ionomer (5 wt. % in water and ethanol, Aldrich) were ultrasonically suspended in a mixture of deionised water and isopropanol (volume ratio 1:1), with catalyst:ionomer ratio z 3:1. Then, this catalyst ink was coated onto a glassy carbon disk electrode (geometric area of 0.0707 cm2) with a micropipette and dried beneath an infrared lamp for 5 min. The catalyst loading was 0.2 mg RuO2 cm2 (i.e. 1 mg RuO2/ATO cm2). Another two working electrodes with ATO (0.8 mg cm2) and unsupported RuO2 (0.2 mg cm2) were investigated for comparing. Cyclic voltammetry was done with an Epsilon potentiostat (BASi, Epsilon_ EC_Ver 1.60.70) at 25 C.
2.4.
PEMWE measurement
The membrane electrode assembly (MEA) was prepared with the catalyst coated on membrane (CCM) method, as described in reference [10], with a Nafion 212 membrane (Dupont), 10 mg cm2 RuO2/ATO (containing c.a. 2 mg cm2 RuO2) on anode, and 1 mg cm2 Pt/C (50 wt.% Pt, Alfa Aesar) on cathode. A DC power supply (Thurlby Thandar Instruments PL330) was applied to provide the voltage. Deionised water, which had been pre-heated to 60 C in the inlet pipes, was pumped through the cell at 0.1e0.2 bar (gauge) pressure at a flow rate of 1e3 cm3 min1. The single cell was maintained at 80 C.
3.
Results and discussion
3.1.
Morphology
SEM images of ATO and RuO2/ATO are shown in Fig. 1. The size of ATO particles was approximately 30e40 nm, which increased to approximately 40e50 nm with RuO2 deposits, as shown in Fig. 1 (A) and (B). Fig. 1(C) and (D) shows the enlarged SEM (150,000 magnification), in which RuO2/ATO particles appeared like spheres with several smaller particles deposited their surfaces, which were presumably RuO2 particles. Unlike unsupported RuO2 nanoparticles, which are usually agglomerates with mudlike or sheet-like morphologies [14,16], the RuO2 supported on ATO were distinctive particles with particle size 10e15 nm. This indicates that with the dispersion effects of ATO, RuO2/ATO may have better surface exposure than unsupported RuO2. Both the ATO and the RuO2 particles exhibited even size distributions. The EDX spectrum of RuO2/ATO was shown in Fig. 1(E), which clearly displayed peaks representing Ru, O, and Sn elements. From semiquantitative measurements of the EDX spectrum, RuO2 was
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 5 8 0 6 e5 8 1 0
Fig. 1 e SEM of ATO and RuO2/ATO: (A) RuO2/ATO, 150003 (B) ATO, 150003 (C) RuO2/ATO, 1500003 (D) ATO, 1500003 (E) EDX spectrum of RuO2/ATO.
found 19.3%e23.7% in the total weights of RuO2/ATO, which agreed with designed ratio of syntheses, approximately 20 wt.%.
3.2.
XRD
The crystal structures of ATO and RuO2/ATO were compared. As shown in Fig. 2, XRD patterns of ATO exhibited a typical tetragonal structure (tin stone SnO2, ICDD 00-041-1145). In the XRD patterns of RuO2/ATO, the peaks representing RuO2 were distinctive (rutile RuO2, ICDD 00-040-1290), although minor overlapping was found between peaks of RuO2 and SnO2. The
inset figure in Fig. 2 is the fitting of (110) peaks of ATO and RuO2 deposits on them. From the peak fittings and Scherrer Equation, average crystalline sizes of ATO and RuO2 were respectively about 32.3 nm and 13.7 nm, which were in accordance with the SEMs (Fig. 1). The peak positions of ATO were identical in the samples with and without RuO2, which agrees that the bulk structure of ATO might hardly be influenced during the deposition of RuO2. Moreover, ATO with RuO2 deposits exhibited slightly weaker peak intensity than the ATO sample, which might be a consequence of the RuO2 impregnation.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 5 8 0 6 e5 8 1 0
Fig. 4 e IeV curve of PEMWE with RuO2/ATO.
Fig. 2 e XRD of ATO and RuO2/ATO.
3.3.
Electrochemical investigation
Fig. 3 shows cyclic voltammograms of electrodes with RuO2/ ATO, ATO, and unsupported RuO2. Current densities of the 0.8 mg cm2 ATO electrode were at least two orders of magnitude smaller than the RuO2/ATO electrode, indicating the voltammetric currents of RuO2/ATO were mainly attributed to the RuO2 deposits. ATO was found a relatively inert support material [17]. The RuO2/ATO electrode demonstrated typical redox behaviors [6,14,16] of RuO2 electrodes in the investigated potential range. However, with the same amount of RuO2, RuO2/ATO exhibited higher voltammetric charge than unsupported RuO2. Since the two samples were synthesized with identical procedures, this enhancement may be considered an evidence of positive effects of using ATO, such as reducing agglomeration and increasing electron conduction of RuO2 catalysts. Actually, the off-peak potential for oxygen evolution on the RuO2/ATO electrode was about 50 mV more negative than unsupported RuO2, as shown in Fig. 3.
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3.4.
PEMWE performance
Fig. 4 shows the steady current-voltage performance of a PEMWE single cell with a RuO2/ATO anode and Nafion 212 membrane. Water electrolysis started at cell potential about 1.3 V which is more negative than values of normal PEMWE [7], which might be due to enhanced catalyst activity. An exponential region of the IeV curve was seen from 1.3 to 1.45 V. Above this potential the curve was almost linear. This behavior is in accordance with RuO2 based OER catalysts [4]. In the linear region, the MEA exhibited a small resistance, which is reasonable because the Nafion 212 membrane is very thin and the RuO2/ATO nanoparticles may lead to a small catalyst/polymer interfacial resistance [10]. Notably, at 1000 mA cm2, the voltage was only 1.56 V.
4.
Conclusions
This work demonstrated that Sb-doped SnO2 (ATO) nanoparticles are promising support materials of OER catalysts for PEMWEs. With a novel colloid method, 20 wt.% RuO2 particles (10e15 nm) were finely deposited on ATO particles (30e40 nm), as shown in SEM and XRD results. This RuO2/ATO catalyst exhibited better activity than unsupported RuO2 for oxygen evolution. A PEMWE single cell using 10 mg cm2 20 wt.% RuO2/ATO achieved 1.56 V at 1000 mA cm2 at 80 C. Besides, good PEMWE performances indicated that using ATO as a support material is a promising topic for OER catalysts. This paper takes RuO2 only as an initial example. Further investigations are required to elucidate the positive effects of ATO in future studies. The novel colloid method may be also useful to synthesis IrO2. More stable catalysts supported on ATO will be reported consequently.
Acknowledgement Fig. 3 e Cyclic voltammogram of RuO2/ATO, 22 C, 20 mV sL1.
The authors want to acknowledge DIUS (UK) and China Scholarship Council (China) for the “UK/China Scholarship for Excellence Scheme”, which sponsored Mr. Xu Wu’s PhD study.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 5 8 0 6 e5 8 1 0
references [10] [1] Takenaka H, Torikai E, Kawami Y, Wakabayashi N. Solid polymer electrolyte water electrolysis. International Journal of Hydrogen Energy 1982;7(5):397e403. [2] Janjua MBI, Le Roy RL. Electrocatalyst performance in industrial water electrolysers. International Journal of Hydrogen Energy 1985;10(1):11e9. [3] Balej J. Electrocatalysts for oxygen evolution in advanced water electrolysis. International Journal of Hydrogen Energy 1985;10(2):89e99. [4] Trasatti S. Physical electrochemistry of ceramic oxides. Electrochimica Acta 1991;36(2):225e41. [5] Marshall A, Børresen B, Hagen G, Tsypkin M, Tunold R. Hydrogen production by advanced proton exchange membrane (PEM) water electrolysers e Reduced energy consumption by improved electrocatalysis. Energy 2007;32 (4):431e6. [6] Song S, Zhang H, Ma X, Shao Z, Baker RT, Yi B. Electrochemical investigation of electrocatalysts for the oxygen evolution reaction in PEM water electrolyzers. International Journal of Hydrogen Energy 2008;33(19): 4955e61. [7] Rasten E, Hagen G, Tunold R. Electrocatalysis in water electrolysis with solid polymer electrolyte. Electrochimica Acta 2003;48(25e26):3945e52. [8] Slavcheva E, Radev I, Bliznakov S, Topalov G, Andreev P, Budevski E. Sputtered iridium oxide films as electrocatalysts for water splitting via PEM electrolysis. Electrochimica Acta 2007;52(12):3889e94. [9] Marshall AT, Sunde S, Tsypkin M, Tunold R. Performance of a PEM water electrolysis cell using IrxRuyTazO2
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
electrocatalysts for the oxygen evolution electrode. International Journal of Hydrogen Energy 2007;32(13):2320e4. Wu X, Scott K. The effects of ionomer content on PEM water electrolyser membrane electrode assembly performance. International Journal of Hydrogen Energy 2010;35(21): 12029e37. Ma L, Sui S, Zhai Y. Preparation and characterization of Ir/TiC catalyst for oxygen evolution. Journal of Power Sources 2008; 177(2):470e7. Siracusano S, Baglio V, D’Urso C, Antonucci V, Arico AS. Preparation and characterization of titanium suboxides as conductive supports of IrO2 electrocatalysts for application in SPE electrolysers. Electrochimica Acta 2009;54(26):6292e9. De Pauli CP, Trasatti S. Electrochemical surface characterization of IrO2 þ SnO2 mixed oxide electrocatalysts. Journal of Electroanalytical Chemistry 1995;396(1e2):161e8. Gaudet J, Tavares AC, Trasatti S, Guay D. Physicochemical characterization of mixed RuO2-SnO2 solid solutions. Chemistry of Materials 2005;17(6):1570e9. Hutchings R, Mu¨ller K, Kotz R, Stucki S. A structural investigation of stabilized oxygen evolution catalysts. Journal of Materials Science 1984;19(12):3987e94. Ito M, Murakami Y, Hayato K, Hiroyuki O, Kiyochika Y, Yoshio T. Preparation of ultrafine RuO2-SnO2 binary oxide particles by a sol-gel process. Journal of the Electrochemical Society 1994;141(5):1242e5. Lee K-S, Park I-S, Cho YH, Jung DS, Jung N, Park HY, et al. Electrocatalytic activity and stability of Pt supported on Sbdoped SnO2 nanoparticles for direct alcohol fuel cells. Journal of Catalysis 2008;258(1):143e52. Marshall AT, Haverkamp RG. Electrocatalytic activity of IrO2RuO2 supported on Sb-doped SnO2 nanoparticles. Electrochimica Acta 2010;55(6):1978e84.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
RuO2 supported on Sb-doped SnO2 nanoparticles for polymer electrolyte membrane water electrolysers Xu Wu, Keith Scott* School of Chemical Engineering and Advanced Materials, Merz Court, Newcastle University, Newcastle upon Tyne, NE17RU, UK
article info
abstract
Article history:
With a colloid method, RuO2 was deposited on Sb-doped SnO2 nanoparticles (ATO, Aldrich,
Received 29 September 2010
30e40 nm), which was employed as a novel support material for anode catalysts of poly-
Accepted 29 October 2010
mer electrolyte membrane water electrolysers (PEMWE). Distinctive RuO2 nanoparticles
Available online 3 April 2011
(10e15 nm) were stably deposited on ATO nanoparticles, which were characterized with XRD and SEM. RuO2/ATO exhibited higher activity than unsupported RuO2 for oxygen
Keywords:
evolution. A PEMWE single cell with 10 mg cm2 20 wt.% RuO2/ATO achieved 1.56 V at
Water electrolyser
1 A cm2 at 80 C.
Electrolysis
Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Oxygen evolution reaction
reserved.
PEMWE RuO2 Sb-doped SnO2
1.
Introduction
The anode catalyst has been the focus of many studies on polymer electrolyte membrane water electrolysers (PEMWE), because that the oxygen evolution reaction (OER) is widely considered the largest cause of overpotential. Typically only those materials based upon expensive RuO2 and IrO2 are regarded as suitable (stable and active) catalysts for OER in acidic medium [1e10]. One method to achieve better performances and lower noble metal use is to utilize catalyst supports, which may increase the dispersion and surface exposure of catalysts, i.e. a substantial increase in catalyst utilization [11]. Also support materials could increase electronic conductivity of OER catalysts, which are usually semiconductive oxides [11,12], and increase catalyst stability [13,14]. Typically, carbon has been used as catalyst support in applications such as polymer electrolyte membrane fuel cells.
However, the electrochemical oxidation of carbon during oxygen evolution in the PEMWE will limit the practical lifetime of catalyst supported on carbon. Materials such as TiC [11], TiO2 [12], SnO2 [13,14] etc. have been demonstrated as supports of oxygen evolution catalysts. It’s commonly found that RuO2 exhibits less overpotential than IrO2. Moreover, RuO2 is several times cheaper than IrO2. Therefore, RuO2 is the most widely employed OER catalyst of dimensionally stable anodes (DSA) in the chlor-alkali industry. Due to their unstable octavalent intermediates, RuO2 electrodes might have insufficient stability. However, mixed oxides such as RuO2eSnO2 have exhibited much more stable performances than RuO2 [14e16]. In this case, SnO2 particles function as both the support material and stabilizer. Since SnO2 is a semiconductor, it’s a reasonable view that properties of RuO2 supported by SnO2 can be improved by enhancing the electronic conductivity of the catalyst support. With this consideration, Sb
* Corresponding author. Tel.: þ44 (0)1912228771; fax: þ44 (0)1912225292. E-mail address: [email protected] (K. Scott). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.10.098
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 5 8 0 6 e5 8 1 0
(V) doped SnO2 (ATO) could be a good alternative, since it exhibits higher electronic conductivity than SnO2, attributing to pentavalent Sb doping. ATO nanoparticles have been recently reported as a promising support material for Pt [17] for fuel cells. More recently, RuO2eIrO2 supported on ATO was studied and reported as a potentially promising catalyst for water electrolysers [18]. However, there are no published performances of PEMWE using ATO nanoparticles as support of the anode catalysts. In this communication, RuO2 was deposited on ATO nanoparticles by a novel modified colloid method. The activity of RuO2/ATO in a PEMWE single cell was demonstrated.
2.
Experimental
2.1.
Catalyst preparation
Ruthenium chloride hydrate (RuCl3$H2O, Alfa Aesar) was dissolved in an aqueous solution (40e50 cm3) containing 1.05 103 mol NaOH, with stoichiometric ratio Ru:NaOH z 1:7. With stirring and N2 bubbling, the solution turned to light green after several hours. Consequently, 80 mg ATO powders (Aldrich, 10 mol% Sb, 30 nm) were added into the solution, so that the final weight ratio of RuO2:ATO z 1:4. Then, after being ultrasonicated for 5 min, the pH value of this solution was adjusted to about 8.0 by slowly titration of a 0.1 mol dm3 HClO4 solution. The solution eventually turned achromatous when a precipitate was obtained after four days. The following reactions happened in this preparation procedure: 3 RuCl3 þ 6OH / RuðOHÞ6 þ3Cl
(1)
3 RuðOHÞ6 þ3Hþ /RuðOHÞ3 ðH2 OÞ3 Y
(2)
7Cl þ ClO4 þ 8Hþ /4Cl2 [ þ 4H2 O
(3)
The hydroxyls in the alkaline solution would substitute chlorions of RuCl3$H2O and a [Ru(OH)6]3 ion would form due to sp3d2 hybridization of Ru3þ, as shown in chemical equation (1). When HClO4 was titrated into the solution, the precipitation of electroneutral Ru(OH)3(H2O)3 and volatilization of chlorine would be the dominant reactions, as equations (2) and (3). The size of Ru(OH)3(H2O)3 could be reduced by slowing the precipitation process. The resulting precursor of Ru(OH)3(H2O)3/ATO was separated with a centrifuge, washed with deionised water several times, and dried at 70 C overnight. Finally, the precursor was heated to 450 C in air with a heating rate of 3 C min1, and then annealed at 450 C for 2 h. During the heattreatment process, Ru(OH)3(H2O)3 would be oxidized to RuO2 and excess water would evaporate, as equation (4). 4RuðOHÞ3 ðH2 OÞ3 =ATO þ O2 / 4RuO2 =ATO þ 18H2 O[
(4)
Another sample RuO2 without ATO was prepared with the same procedure for comparing.
2.2.
Characterizations
The sample RuO2/ATO and ATO powders were analyzed with X-ray diffraction (XRD, PANalytical X’Pert Pro Diffractometer), scanning electron microscopy (SEM, JEOL JSM5300LV), and
5807
Energy dispersive X-ray spectroscopy (EDX, Rontec Quantax) attached to SEM. Phase identification was carried out by X’Pert High Score software and the ICDD database PDF2-2004.
2.3.
Electrochemical investigation
Electrochemical experiments were carried out in a 3-electrode cell in a 0.5 mol dm3 H2SO4 solution prepared with deionised water. A saturated calomel electrode reference electrode (SCE) was connected to a Luggin capillary which was positioned close to the working electrode. The counter electrode was a Pt foil (1 cm 2.5 cm). For preparing working electrodes, the RuO2/ATO (of which RuO2 is approximately 20% of total weight) and Nafion ionomer (5 wt. % in water and ethanol, Aldrich) were ultrasonically suspended in a mixture of deionised water and isopropanol (volume ratio 1:1), with catalyst:ionomer ratio z 3:1. Then, this catalyst ink was coated onto a glassy carbon disk electrode (geometric area of 0.0707 cm2) with a micropipette and dried beneath an infrared lamp for 5 min. The catalyst loading was 0.2 mg RuO2 cm2 (i.e. 1 mg RuO2/ATO cm2). Another two working electrodes with ATO (0.8 mg cm2) and unsupported RuO2 (0.2 mg cm2) were investigated for comparing. Cyclic voltammetry was done with an Epsilon potentiostat (BASi, Epsilon_ EC_Ver 1.60.70) at 25 C.
2.4.
PEMWE measurement
The membrane electrode assembly (MEA) was prepared with the catalyst coated on membrane (CCM) method, as described in reference [10], with a Nafion 212 membrane (Dupont), 10 mg cm2 RuO2/ATO (containing c.a. 2 mg cm2 RuO2) on anode, and 1 mg cm2 Pt/C (50 wt.% Pt, Alfa Aesar) on cathode. A DC power supply (Thurlby Thandar Instruments PL330) was applied to provide the voltage. Deionised water, which had been pre-heated to 60 C in the inlet pipes, was pumped through the cell at 0.1e0.2 bar (gauge) pressure at a flow rate of 1e3 cm3 min1. The single cell was maintained at 80 C.
3.
Results and discussion
3.1.
Morphology
SEM images of ATO and RuO2/ATO are shown in Fig. 1. The size of ATO particles was approximately 30e40 nm, which increased to approximately 40e50 nm with RuO2 deposits, as shown in Fig. 1 (A) and (B). Fig. 1(C) and (D) shows the enlarged SEM (150,000 magnification), in which RuO2/ATO particles appeared like spheres with several smaller particles deposited their surfaces, which were presumably RuO2 particles. Unlike unsupported RuO2 nanoparticles, which are usually agglomerates with mudlike or sheet-like morphologies [14,16], the RuO2 supported on ATO were distinctive particles with particle size 10e15 nm. This indicates that with the dispersion effects of ATO, RuO2/ATO may have better surface exposure than unsupported RuO2. Both the ATO and the RuO2 particles exhibited even size distributions. The EDX spectrum of RuO2/ATO was shown in Fig. 1(E), which clearly displayed peaks representing Ru, O, and Sn elements. From semiquantitative measurements of the EDX spectrum, RuO2 was
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 5 8 0 6 e5 8 1 0
Fig. 1 e SEM of ATO and RuO2/ATO: (A) RuO2/ATO, 150003 (B) ATO, 150003 (C) RuO2/ATO, 1500003 (D) ATO, 1500003 (E) EDX spectrum of RuO2/ATO.
found 19.3%e23.7% in the total weights of RuO2/ATO, which agreed with designed ratio of syntheses, approximately 20 wt.%.
3.2.
XRD
The crystal structures of ATO and RuO2/ATO were compared. As shown in Fig. 2, XRD patterns of ATO exhibited a typical tetragonal structure (tin stone SnO2, ICDD 00-041-1145). In the XRD patterns of RuO2/ATO, the peaks representing RuO2 were distinctive (rutile RuO2, ICDD 00-040-1290), although minor overlapping was found between peaks of RuO2 and SnO2. The
inset figure in Fig. 2 is the fitting of (110) peaks of ATO and RuO2 deposits on them. From the peak fittings and Scherrer Equation, average crystalline sizes of ATO and RuO2 were respectively about 32.3 nm and 13.7 nm, which were in accordance with the SEMs (Fig. 1). The peak positions of ATO were identical in the samples with and without RuO2, which agrees that the bulk structure of ATO might hardly be influenced during the deposition of RuO2. Moreover, ATO with RuO2 deposits exhibited slightly weaker peak intensity than the ATO sample, which might be a consequence of the RuO2 impregnation.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 5 8 0 6 e5 8 1 0
Fig. 4 e IeV curve of PEMWE with RuO2/ATO.
Fig. 2 e XRD of ATO and RuO2/ATO.
3.3.
Electrochemical investigation
Fig. 3 shows cyclic voltammograms of electrodes with RuO2/ ATO, ATO, and unsupported RuO2. Current densities of the 0.8 mg cm2 ATO electrode were at least two orders of magnitude smaller than the RuO2/ATO electrode, indicating the voltammetric currents of RuO2/ATO were mainly attributed to the RuO2 deposits. ATO was found a relatively inert support material [17]. The RuO2/ATO electrode demonstrated typical redox behaviors [6,14,16] of RuO2 electrodes in the investigated potential range. However, with the same amount of RuO2, RuO2/ATO exhibited higher voltammetric charge than unsupported RuO2. Since the two samples were synthesized with identical procedures, this enhancement may be considered an evidence of positive effects of using ATO, such as reducing agglomeration and increasing electron conduction of RuO2 catalysts. Actually, the off-peak potential for oxygen evolution on the RuO2/ATO electrode was about 50 mV more negative than unsupported RuO2, as shown in Fig. 3.
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3.4.
PEMWE performance
Fig. 4 shows the steady current-voltage performance of a PEMWE single cell with a RuO2/ATO anode and Nafion 212 membrane. Water electrolysis started at cell potential about 1.3 V which is more negative than values of normal PEMWE [7], which might be due to enhanced catalyst activity. An exponential region of the IeV curve was seen from 1.3 to 1.45 V. Above this potential the curve was almost linear. This behavior is in accordance with RuO2 based OER catalysts [4]. In the linear region, the MEA exhibited a small resistance, which is reasonable because the Nafion 212 membrane is very thin and the RuO2/ATO nanoparticles may lead to a small catalyst/polymer interfacial resistance [10]. Notably, at 1000 mA cm2, the voltage was only 1.56 V.
4.
Conclusions
This work demonstrated that Sb-doped SnO2 (ATO) nanoparticles are promising support materials of OER catalysts for PEMWEs. With a novel colloid method, 20 wt.% RuO2 particles (10e15 nm) were finely deposited on ATO particles (30e40 nm), as shown in SEM and XRD results. This RuO2/ATO catalyst exhibited better activity than unsupported RuO2 for oxygen evolution. A PEMWE single cell using 10 mg cm2 20 wt.% RuO2/ATO achieved 1.56 V at 1000 mA cm2 at 80 C. Besides, good PEMWE performances indicated that using ATO as a support material is a promising topic for OER catalysts. This paper takes RuO2 only as an initial example. Further investigations are required to elucidate the positive effects of ATO in future studies. The novel colloid method may be also useful to synthesis IrO2. More stable catalysts supported on ATO will be reported consequently.
Acknowledgement Fig. 3 e Cyclic voltammogram of RuO2/ATO, 22 C, 20 mV sL1.
The authors want to acknowledge DIUS (UK) and China Scholarship Council (China) for the “UK/China Scholarship for Excellence Scheme”, which sponsored Mr. Xu Wu’s PhD study.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 5 8 0 6 e5 8 1 0
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