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Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis
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Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis Sang-Beom Han a,b, Yong-Hwan Mo b, Yong-Soo Lee b, Seul-Gi Lee a, Deok-Hye Park a, Kyung-Won Park a,* a b
Department of Chemical Engineering, Soongsil University, Seoul, 06978, Republic of Korea Boyaz Energy, 165 Gasandigital 2-ro, Geumcheon-gu, Seoul, 08504, Republic of Korea
highlights
graphical abstract
IrO2 and ATO nanostructures were synthesized
with
the
Adams
fusion method. The samples showed a well-defined porous
high-crystalline
nanostructure. ATO
nanoparticles
rounded
by
IrO2
were
sur-
nanoparticles
without agglomeration. IrO2/ATO electrodes showed the superior OER properties. The improved activity results from favorable mass transfer of a reactant of the mesoporous nanostructure.
article info
abstract
Article history:
In proton exchange membrane (PEM) water electrolysis, iridium oxide (IrO2) has often been
Received 18 September 2019
utilized as a main catalyst for the oxygen evolution reaction (OER) as a rate-determining
Received in revised form
step. In general, the performance of PEM water electrolysis is dominantly affected by the
23 October 2019
specific surface area and the porous structure of the IrO2 catalyst. Thus, in this study, IrO2
Accepted 17 November 2019
and antimony-doped tin oxide (ATO) nanostructures with high specific surface areas were
Available online xxx
synthesized through the Adams fusion method. The as-prepared samples showed welldefined porous high-crystalline nanostructures. The ATO nanoparticles as a support
Keywords:
were surrounded by IrO2 nanoparticles as a catalyst without serious agglomeration, indi-
Iridium oxide
cating that the IrO2 catalyst was uniformly distributed on the ATO support. Compared to
Sb-doped SnO2
* Corresponding author. E-mail address: [email protected] (K.-W. Park). https://doi.org/10.1016/j.ijhydene.2019.11.109 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Han S-B et al., Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.109
2
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Oxygen evolution reaction
pure IrO2, the IrO2/ATO mixture electrodes showed superior OER properties because of
Polymer electrolyte membrane
their increased electrochemical active sites.
Water electrolysis
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen generation can be realized using a variety of approaches, including water electrolysis, natural gas reforming, the partial oxidation of fossil fuels, and the gasification of biomasses [1,2]. Among these approaches, water electrolysis is known as an eco-friendly way to produce clean H2 as an energy carrier [3]. In water electrolysis, proton exchange membrane (PEM), alkaline, and ceramic-based H2 generation methods have been proposed [4]. PEM water electrolysis in particular exhibits a number of advantages, such as high efficiency, high hydrogen production rate, the production of high purity hydrogen and oxygen, rapid system response, and high safety [5]. In PEM water electrolysis, iridium oxide has been utilized as a main catalyst for oxygen evolution reaction (OER) as a rate-determining step [6]. In general, the performances of electrochemical devices are mainly affected by the specific surface area and the porous structure of catalysts [7e9]. Thus, iridium oxide catalysts with high specific surface areas and porous nanostructures need to be prepared for high-performance water electrolysis [10]. Iridium oxide has been synthesized using various preparation methods, such as hydrolysis, chemical vapor deposition, the colloid method, ultrasonic spray pyrolysis, and the molten salt method [11,12]. In particular, an iridium oxide nanoparticle prepared using the colloid method exhibited a fairly high specific surface area of ~200 m2 g1 [13]. However, for high-performance electrolysis, an iridium oxide anode nanostructure catalyst with a significantly high specific surface area needs to be synthesized [14]. Herein, we simply prepared a well-defined porous iridium oxide (IrO2) nanostructure with a high specific surface area using the Adams fusion method without any templates and/or additives [15]. In addition, an antimony-doped tin oxide (ATO) nanostructure as a support was synthesized using the Adams fusion method [16,17]. Powder mixtures of IrO2 catalyst and ATO support with different weight ratios (IrO2/ATO) were prepared using a ball milling method. Compared to pure IrO2, the IrO2/ATO electrodes showed superior OER properties due to the increased electrochemical active sites. The mesoporous nanostructure and high specific surface areas of the electrodes can affect the catalytic activity because of the favorable mass transfer of a reactant.
Experimental section Synthesis of iridium oxide and Sb-doped SnO2 Iridium oxide and antimony-doped tin oxide (ATO) were synthesized using the Adams fusion method [15,18]. The
desired amount of IrCl3 was dissolved into 150 mL de-ionized (DI) water at room temperature. Next, excessive NaNO3 was added into the solution under sonication with stirring for 30 min. The solvent in the solution was then evaporated at 80 C for 4 h. The remaining powder was calcined under an air atmosphere at 450 C for 40 min. The calcined black powder was washed and filtered with 0.5 M HCl solution, and dried at 50 C for 12hrs. To prepare ATO, the desired amount of SnCl4,5H2O was dissolved into 100 mL D.I. water with stirring for 30 min, and then 5% SbCl3 was added, which was calculated based on tin chloride, and the solution was continually stirred for 24 h. Then, excessive NaNO3 was added into the solution under sonication with stirring for 30 min. The solvent in the solution was evaporated at 80 C for 4 h. The remaining powder was calcined under an air atmosphere at 450 C for 40 min. Next, the calcined black powder was washed and filtered with 0.5 M HCl solution, then dried at 50 C for 12 h. Finally, the powder mixtures of IrOx and ATO with weight ratios of 7:3, 5:5 and 3:7 were placed into 300 mL Teflon bottles with zirconia balls and then homogenized using a ball mill machine (Changsha Deco Equipment Co., Ltd) at 150 rpm for 1 h.
Structural characterization The crystalline structures of the samples were analyzed by Xray Diffraction (XRD, Bruker, D2 Phase system). The particle sizes and structures of the samples were identified using highresolution transmission electron microscope (HR-TEM, JEOL, JEM-F200) operating at 200 kV. Energy dispersive X-ray spectroscopy (EDS, JEOL, JEM-F200) analysis was conducted to obtain the distribution of metal oxide sites such as Ir, Sn, Sb, and O. The N2 adsorption and desorption isotherms of the samples were measured at 77 K. Prior to measurement, all of the samples were degassed at 350 C for 4 h under vacuum condition. The specific surface areas of the samples were calculated using the Brunauer-Emmett-Teller (BET) equation with an N2 adsorption range from 0.01 to 0.3 relative pressure.
Electrochemical characterization The electrochemical properties of the samples were characterized using a potentiostat (Metrohm Autolab, PGSTAT302N) in a three-electrode electrochemical cell at room temperature. A graphite rod and Ag/AgCl (in 3 M KCl) were used as the counter and reference electrodes, respectively. The working electrode was prepared by loading the prepared catalysts on the glassy carbon (GC) electrodes of the rotating disk electrode (RDE) system. The catalyst ink was prepared by mixing the asprepared catalyst powder with DI water, isopropyl alcohol (IPA, Sigma Aldrich), and Nafion ionomer solution (5 wt% Nafion® ionomer, Sigma Aldrich) and 10 mL of the catalyst ink
Please cite this article as: Han S-B et al., Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.109
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Preparation of membrane-electrode-assembly (MEA) and single cell test The MEAs with an active area of 5 cm2 were prepared using catalyst-coated membranes (CCMs). The catalyst ink for the oxygen anode was prepared using a paste mixer (200 rpm, 30 min) and under sonication (10 min) with the as-prepared catalyst powder with DI water, ethanol (Sigma Aldrich), and Nafion ionomer solution (5 wt% Nafion® ionomer, Sigma Aldrich). The catalyst ink for the oxygen anode was coated on a Nafion membrane (211, Dupont) and the other side of the membrane was coated with a commercial Pt/C (40% Pt on Carbon paper, Fuel Cell Earth) for a hydrogen cathode through an ultrasonication spraying equipment (BE-CT-1901-400, Boyaz Energy Co.). The loading amounts of Pt/C and the asprepared samples were 5 and 0.5 mg cm2, respectively. Following the catalyst spraying, 250 mL of 5 wt% Nafion ionomer was additionally sprayed onto the electrodes, then dried on an 80 C heated vacuum plate for 2 min. The MEAs were fabricated using a hot press at 120 C under a pressure of 2 MPa for 2.5 min. A single cell with an active area of 5 cm2 was assembled with the as-prepared MEA, gas diffusion layer (SGL 39BC, SGL Carbon ltd.), and current collector (Pt-coated titanium plate). The performance of the single cell was measured using a computer-controlled power supply system (Boyaz Energy Co.) at 80 C under a DI water flow rate of 3 mL min1.
Results and discussion Fig. 1 shows the XRD patterns of pure IrO2, pure ATO, and power mixtures of IrO2 and ATO (Ir/A-70, Ir/A-50, Ir/A-30)
Intensity (a.u.)
Ir/A-70 Ir/A-50 Ir/A-30 ATO
20
was dropped on the GC electrode. After being dried in a 50 C oven for 10 min, the loading amount of the catalyst deposited on the GC electrode was ~200 mg cm2. The electrochemical performances of the samples were evaluated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). All of the electrode potentials were converted to reversible hydrogen electrode (RHE) using the Nernst equation.
Quantity Adsorbed (cm3/g STP)
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0.0
Ir/A-70 Ir/A-50 Ir/A-30 ATO
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0) Fig. 2 e N2 adsorption and desorption isotherms of pure IrO2, IrO2/ATO mixtures, and pure ATO at 77 K. prepared using a ball milling method. The sample name of IrO2/ATO mixture has defined as Ir/A-x. Ir, A, and x represented into IrO2, ATO, and ratio of IrO2/ATO, respectively (i.e. Ir/A-70 ¼ 7:3 IrO2/ATO). The pure IrOx sample showed characteristic peaks at 28.1 , 34.6 , 39.3 , 54.2 , 58.5 , and 66.1 , which corresponded to the (110), (101), (200), (211), (220), and (310) planes, respectively, of the rutile IrO2 structure (JCPDS Cards No. 43e1019, P42/mnm point group) [19]. The pure ATO sample showed characteristic peaks at 26.6 , 33.8 , 38.1 , and 51.8 corresponding to the (110), (101), (200), and (211) planes, respectively (JCPDS Cards No. 88e2348) [20]. Furthermore, using the Debye-Scherrer equation, the average particle sizes of IrO2 and ATO were determined to be ~3.2 and ~4.9 nm, respectively. In particular, as the amount of ATO increased in the powder mixtures, the peaks corresponding to the (110) plane were shifted to those corresponding to the pure ATO and the intensities of the (211) plane gradually increased [15]. This indicates that the crystal structures of IrO2 and ATO were maintained following the ball milling process. Fig. 2 shows the N2 adsorption/desorption isotherms of the as-prepared pure IrO2, ATO, Ir/A-70, Ir/A-50, and Ir/A-30. The pure IrO2 and ATO samples exhibited H4 type and IV hysteresis loops, respectively, indicating porous structures with fairly high specific surface areas of ~214 and ~131 m2 g1, respectively. According to the literature, typically, IrO2 and
Table 1 e Textural properties of pure IrO2, IrO2/ATO mixtures, and pure ATO. Catalyst IrO2 Ir/A-70 Ir/A-50 Ir/A-30 ATO
10
20
30
40
50
60
70
80
2 theta (degree) Fig. 1 e XRD patterns of pure IrO2, IrO2/ATO mixtures, and pure ATO.
a b
c
BET surface areaa (m2/g)
Pore volumeb (cm3/g)
Pore Diameterc (nm)
214 165 144 121 131
0.13 0.16 0.13 0.12 0.12
2.1 3.8 3.8 3.9 3.3
Calculated using BET equation. Pore volume evaluated from the N2 adsorption-desorption isotherms. Peak pore size determined from BJH desorption branch.
Please cite this article as: Han S-B et al., Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.109
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Fig. 3 e Pore size distribution of pure IrO2, IrO2/ATO mixtures, and pure ATO at (a) adsorption and (b) desorption branches.
ATO nanostructures had relatively lower specific surface areas than those of the samples synthesized in the present study [18,21e25]. The high specific surface areas of IrO2 and ATO as a catalyst and a support, respectively, can affect the catalytic activity because of favorable mass transfer of a reactant. With a decreasing weight ratio of IrO2, the specific
surface areas of Ir/A-70, Ir/A-50, and Ir/A-30 decreased from 165 to 121 m2 g1, and the pore volumes of the powder mixtures decreased from 0.16 to 0.12 cm3 g1 (Table 1). Based on the BJH adsorption branch, the main pore diameters of pure IrO2 and ATO were determined to be 2.1and 3.3 nm, respectively. The IrO2/ATO mixtures of Ir/A-70, Ir/A-50, and Ir/A-30 showed pore diameters of 3.8, 3.8, and 3.9 nm, respectively, which were determined based on the BJH desorption branch. Fig. 3 shows the pore size distributions of the as-prepared pure IrO2, pure ATO, and IrO2/ATO mixtures as measured using the adsorption and the desorption branches. The pure IrO2 and pure ATO were found to have pore diameters of 4.0 and 5.5 nm, respectively. In particular, the distinct peaks in the desorption branches, and not in the adsorption branches, appeared in pore diameters ranging from 3 to 4 nm, indicating a porous structure with a narrow inlet and a wide interior. Fig. 4 shows HR-TEM images of pure IrO2, pure ATO, Ir/A70, Ir/A-50, and Ir/A-30. The pure IrO2 exhibited an average particle size of ~3 nm and a high crystalline structure with an interplanar spacing of 2.9 A corresponding to the dominant {110} planes of the rutile structure (Fig. 4(a) and (f)). Furthermore, the pure ATO showed an average particle size of ~5 nm as well as a high crystalline structure with an interplanar spacing of 1.4 A corresponding to the dominant {110} planes of the rutile structure (Fig. 4(e) and (j)). Thus, the results showed that the pure IrO2 and ATO samples with fairly homogeneous nanostructures were successfully synthesized using the present synthesis method. As shown in the EDS mapping images (Fig. 5), the ATO nanoparticles as a support were surrounded by IrO2 nanoparticles as a catalyst, indicating that the IrO2 catalyst was uniformly distributed on the ATO support without serous agglomeration. Fig. 6(a) shows CVs of the pure IrO2 and IrO2/ATO mixtures measured with a sweeping rate of 50 mV s1 in 0.1 M HClO4 electrolyte solution at 25 C. The active sites of oxide catalysts for electrochemical reactions, i.e., OER, are typically proportional to the value of voltammetric charge (q), which can be measured by integrating the cyclic voltammogram [26e28]. The q values for the oxidation and the reduction reactions
Fig. 4 e HR-TEM images of (a,f) pure IrO2, (b,g) Ir/A-30, (c,h) Ir/A-50, (d,i) Ir/A-70, and (e,j) pure ATO. Please cite this article as: Han S-B et al., Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.109
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Fig. 5 e EDS images of Ir-L, SneK, and SbeK series of the IrO2/ATO mixtures.
were determined by integrating the graphs with regard to the positive and the negative directions, respectively, based on a current density of 0 mA cm2. The q values of pure IrO2, Ir/A70, Ir/A-50, and Ir/A-30 for OER were 28.3, 36.1, 22.1, and 10.9 mC cm2, respectively. The q values of pure IrO2, Ir/A-70, Ir/A-50, and Ir/A-30 for HER were 28.5, 36.2, 22.3, and 11.2 mC cm2, respectively. The total q values of pure IrO2, Ir/ A-70, Ir/A-50, and Ir/A-30 for OER were 56.7, 72.2, 44.2, and 22.1 mC cm2, respectively (Table 2). As mentioned previously, the as-prepared IrO2 showed a specific surface area (~214 m2 g1) four times higher than that of a commercial IrO2 (~50 m2 g1). Compared to the q values, among these samples, Ir/A-70 exhibited the highest q value, i.e., the highest active site for OER. Thus, Ir/A-70 with highest active site is expected to have the best electrocatalytic activity for OER. The points of inflexion were observed through the first derivative curves in the HER (Fig. 6(b)). The currents for hydrogen underpotential deposition (H-UPD) in all of the samples appeared at ~0.23 V vs. RHE. For the pure IrO2, Ir/A-70 and Ir/A-50, the electrochemical reduction reactions of Ir2þ/Ir3þ and Ir3þ/Ir4þ occurred at 0.68 and 0.83 V vs. RHE, respectively. By contrast, Ir/A-30 showed electrochemical reduction reactions of Ir2þ/Ir3þ and Ir3þ/Ir4þ at 0.65 and 0.83 V vs. RHE, respectively. The lower reduction potentials of Ir/A-30 correspond to the increased activation energies for the reduction reactions. Fig. 7 shows LSVs of the pure IrO2 and IrO2/ATO mixtures measured in a potential range from 1.0 to 1.6 V vs. RHE with a sweeping rate of 5 mV s1 in 0.1 M HClO4 electrolyte solution at 25 C. It has been reported that the oxygen evolution reaction theoretically commences at 1.23 V vs. RHE. However, the onset potential of the samples for the OER was found to be
Fig. 6 e (a) Cyclic voltammograms (CVs) and (b) first derivative curves for pure IrO2 and IrO2/ATO mixtures in 0.1 M HClO4 solution at sweeping rate of 50 mV s¡1.
Please cite this article as: Han S-B et al., Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.109
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Table 2 e Electrochemical properties of pure IrO2, IrO2/ATO mixtures, and pure ATO. CVa
Catalyst 2
IrO2 Ir/A-70 Ir/A-50 Ir/A-30 a b
LSVb 2
2
OER mC/cm )
HER (mC/cm )
Total (mC/cm )
Overpotential (V)
28.3 36.1 22.1 10.9
28.5 36.2 22.3 11.2
56.7 72.2 44.4 22.1
0.283 0.256 0.299 0.295
[email protected] V (A/cm2)
I/[email protected] V (mA/mg)
0.799 0.838 0.789 0.578
80 120 158 193
Positive area (OER) and negative area (HER) were calculated by integrating the areas divided at 0 mA/cm2 as y-axis. Overpotential was calculated by difference value from 1.23 V at 10 mA/cm2.
Fig. 7 e Comparison of LSV curves for pure IrO2 and IrO2/ ATO mixtures in 1 M HClO4 electrolyte solution.
~1.4 V, due to the loss of energy (or latent heat) required for the phase transition of H2O [11]. Furthermore, the overpotentials of pure IrO2, Ir/A-70, Ir/A-50, and Ir/A-30 at a current density of 10 mA cm2 were 283, 256, 299, and 295 mV, respectively (Table 2). Fig. 8(a) shows polarization curves of the single cells with pure IrO2 and IrO2/ATO mixtures as anode catalysts and a commercial Pt/C as a cathode catalyst. The current densities of pure Ir/A-70, Ir/A-50, and Ir/A-30 at 1.6 V were 0.799, 0.838, 0.789, and 0.578 A cm2, respectively (Table 2). Generally, the utilization amount of IrO2 catalyst for OER is a crucial consideration in terms of cost of the MEA for PEM water electrolysis. Notably, compared to pure IrO2, Ir/A-70 and Ir/ A-30 showed significantly improved OER performances, despite the fact that the amount of IrO2 was considerably reduced by up to 50 wt%. The superior OER properties of Ir/ A-70 and Ir/A-50 may be attributed to the increased electrochemical active sites, as confirmed by the CV (Fig. 6(a)). The polarization curves of the single cells were normalized with the amount of IrO2 utilized at the cathode (Fig. 8(b)). The normalized current densities of pure IrO2, Ir/A-70, Ir/A50, and Ir/A-30 at 1.6 V were 80, 120, 158, and 193 mA mg2, respectively. As the amounts of ATO increased, the catalytic efficiencies of the catalysts were found to increase. The Ir/A30 catalyst showed the highest normalized current density and the lowest current density at 1.6 V, which were attributed to the absolute deficiency of the IrO2 catalyst in Ir/A-30 mixture.
Fig. 8 e (a) Polarization curves and (b) normalized curves during PEM water electrolysis using pure IrO2 and IrO2/ ATO mixtures (cathode; 20 wt% Pt/C, temperature; 80 C, and electrolyte; Nafion 117).
Conclusions In summary, porous iridium oxide and antimony-doped tin oxide nanostructures with high specific surface areas were synthesized through the Adams fusion method. Compared to pure IrO2, the IrO2/ATO electrodes showed superior OER properties due to the increased electrochemical active sites. The mesoporous nanostructure and high specific surface areas achieved when using IrO2 and ATO as a catalyst and a support, respectively, can affect the catalytic activity in the
Please cite this article as: Han S-B et al., Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.109
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IrO2/ATO electrodes due to the favorable mass transfer of a reactant. [13]
Acknowledgments This research was supported by the Hydrogen Energy Innovation Technology Development Program of the National Research Foundation of Korea (NRF) funded by the Korean government (Ministry of Science and ICT(MSIT)) (No. 2019M3E6A1104186).
[14]
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[28] Rozain C, Mayousse E, Guillet N, Millet P. Influence of iridium oxide loadings on the performance of PEM water electrolysis cells: Part I-Pure IrO2-based anodes. Appl Catal, B 2016;182:153e60. https://doi.org/10.1016/ j.apcatb.2015.09.013.
Please cite this article as: Han S-B et al., Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.109
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Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis Sang-Beom Han a,b, Yong-Hwan Mo b, Yong-Soo Lee b, Seul-Gi Lee a, Deok-Hye Park a, Kyung-Won Park a,* a b
Department of Chemical Engineering, Soongsil University, Seoul, 06978, Republic of Korea Boyaz Energy, 165 Gasandigital 2-ro, Geumcheon-gu, Seoul, 08504, Republic of Korea
highlights
graphical abstract
IrO2 and ATO nanostructures were synthesized
with
the
Adams
fusion method. The samples showed a well-defined porous
high-crystalline
nanostructure. ATO
nanoparticles
rounded
by
IrO2
were
sur-
nanoparticles
without agglomeration. IrO2/ATO electrodes showed the superior OER properties. The improved activity results from favorable mass transfer of a reactant of the mesoporous nanostructure.
article info
abstract
Article history:
In proton exchange membrane (PEM) water electrolysis, iridium oxide (IrO2) has often been
Received 18 September 2019
utilized as a main catalyst for the oxygen evolution reaction (OER) as a rate-determining
Received in revised form
step. In general, the performance of PEM water electrolysis is dominantly affected by the
23 October 2019
specific surface area and the porous structure of the IrO2 catalyst. Thus, in this study, IrO2
Accepted 17 November 2019
and antimony-doped tin oxide (ATO) nanostructures with high specific surface areas were
Available online xxx
synthesized through the Adams fusion method. The as-prepared samples showed welldefined porous high-crystalline nanostructures. The ATO nanoparticles as a support
Keywords:
were surrounded by IrO2 nanoparticles as a catalyst without serious agglomeration, indi-
Iridium oxide
cating that the IrO2 catalyst was uniformly distributed on the ATO support. Compared to
Sb-doped SnO2
* Corresponding author. E-mail address: [email protected] (K.-W. Park). https://doi.org/10.1016/j.ijhydene.2019.11.109 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Han S-B et al., Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.109
2
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Oxygen evolution reaction
pure IrO2, the IrO2/ATO mixture electrodes showed superior OER properties because of
Polymer electrolyte membrane
their increased electrochemical active sites.
Water electrolysis
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen generation can be realized using a variety of approaches, including water electrolysis, natural gas reforming, the partial oxidation of fossil fuels, and the gasification of biomasses [1,2]. Among these approaches, water electrolysis is known as an eco-friendly way to produce clean H2 as an energy carrier [3]. In water electrolysis, proton exchange membrane (PEM), alkaline, and ceramic-based H2 generation methods have been proposed [4]. PEM water electrolysis in particular exhibits a number of advantages, such as high efficiency, high hydrogen production rate, the production of high purity hydrogen and oxygen, rapid system response, and high safety [5]. In PEM water electrolysis, iridium oxide has been utilized as a main catalyst for oxygen evolution reaction (OER) as a rate-determining step [6]. In general, the performances of electrochemical devices are mainly affected by the specific surface area and the porous structure of catalysts [7e9]. Thus, iridium oxide catalysts with high specific surface areas and porous nanostructures need to be prepared for high-performance water electrolysis [10]. Iridium oxide has been synthesized using various preparation methods, such as hydrolysis, chemical vapor deposition, the colloid method, ultrasonic spray pyrolysis, and the molten salt method [11,12]. In particular, an iridium oxide nanoparticle prepared using the colloid method exhibited a fairly high specific surface area of ~200 m2 g1 [13]. However, for high-performance electrolysis, an iridium oxide anode nanostructure catalyst with a significantly high specific surface area needs to be synthesized [14]. Herein, we simply prepared a well-defined porous iridium oxide (IrO2) nanostructure with a high specific surface area using the Adams fusion method without any templates and/or additives [15]. In addition, an antimony-doped tin oxide (ATO) nanostructure as a support was synthesized using the Adams fusion method [16,17]. Powder mixtures of IrO2 catalyst and ATO support with different weight ratios (IrO2/ATO) were prepared using a ball milling method. Compared to pure IrO2, the IrO2/ATO electrodes showed superior OER properties due to the increased electrochemical active sites. The mesoporous nanostructure and high specific surface areas of the electrodes can affect the catalytic activity because of the favorable mass transfer of a reactant.
Experimental section Synthesis of iridium oxide and Sb-doped SnO2 Iridium oxide and antimony-doped tin oxide (ATO) were synthesized using the Adams fusion method [15,18]. The
desired amount of IrCl3 was dissolved into 150 mL de-ionized (DI) water at room temperature. Next, excessive NaNO3 was added into the solution under sonication with stirring for 30 min. The solvent in the solution was then evaporated at 80 C for 4 h. The remaining powder was calcined under an air atmosphere at 450 C for 40 min. The calcined black powder was washed and filtered with 0.5 M HCl solution, and dried at 50 C for 12hrs. To prepare ATO, the desired amount of SnCl4,5H2O was dissolved into 100 mL D.I. water with stirring for 30 min, and then 5% SbCl3 was added, which was calculated based on tin chloride, and the solution was continually stirred for 24 h. Then, excessive NaNO3 was added into the solution under sonication with stirring for 30 min. The solvent in the solution was evaporated at 80 C for 4 h. The remaining powder was calcined under an air atmosphere at 450 C for 40 min. Next, the calcined black powder was washed and filtered with 0.5 M HCl solution, then dried at 50 C for 12 h. Finally, the powder mixtures of IrOx and ATO with weight ratios of 7:3, 5:5 and 3:7 were placed into 300 mL Teflon bottles with zirconia balls and then homogenized using a ball mill machine (Changsha Deco Equipment Co., Ltd) at 150 rpm for 1 h.
Structural characterization The crystalline structures of the samples were analyzed by Xray Diffraction (XRD, Bruker, D2 Phase system). The particle sizes and structures of the samples were identified using highresolution transmission electron microscope (HR-TEM, JEOL, JEM-F200) operating at 200 kV. Energy dispersive X-ray spectroscopy (EDS, JEOL, JEM-F200) analysis was conducted to obtain the distribution of metal oxide sites such as Ir, Sn, Sb, and O. The N2 adsorption and desorption isotherms of the samples were measured at 77 K. Prior to measurement, all of the samples were degassed at 350 C for 4 h under vacuum condition. The specific surface areas of the samples were calculated using the Brunauer-Emmett-Teller (BET) equation with an N2 adsorption range from 0.01 to 0.3 relative pressure.
Electrochemical characterization The electrochemical properties of the samples were characterized using a potentiostat (Metrohm Autolab, PGSTAT302N) in a three-electrode electrochemical cell at room temperature. A graphite rod and Ag/AgCl (in 3 M KCl) were used as the counter and reference electrodes, respectively. The working electrode was prepared by loading the prepared catalysts on the glassy carbon (GC) electrodes of the rotating disk electrode (RDE) system. The catalyst ink was prepared by mixing the asprepared catalyst powder with DI water, isopropyl alcohol (IPA, Sigma Aldrich), and Nafion ionomer solution (5 wt% Nafion® ionomer, Sigma Aldrich) and 10 mL of the catalyst ink
Please cite this article as: Han S-B et al., Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.109
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Preparation of membrane-electrode-assembly (MEA) and single cell test The MEAs with an active area of 5 cm2 were prepared using catalyst-coated membranes (CCMs). The catalyst ink for the oxygen anode was prepared using a paste mixer (200 rpm, 30 min) and under sonication (10 min) with the as-prepared catalyst powder with DI water, ethanol (Sigma Aldrich), and Nafion ionomer solution (5 wt% Nafion® ionomer, Sigma Aldrich). The catalyst ink for the oxygen anode was coated on a Nafion membrane (211, Dupont) and the other side of the membrane was coated with a commercial Pt/C (40% Pt on Carbon paper, Fuel Cell Earth) for a hydrogen cathode through an ultrasonication spraying equipment (BE-CT-1901-400, Boyaz Energy Co.). The loading amounts of Pt/C and the asprepared samples were 5 and 0.5 mg cm2, respectively. Following the catalyst spraying, 250 mL of 5 wt% Nafion ionomer was additionally sprayed onto the electrodes, then dried on an 80 C heated vacuum plate for 2 min. The MEAs were fabricated using a hot press at 120 C under a pressure of 2 MPa for 2.5 min. A single cell with an active area of 5 cm2 was assembled with the as-prepared MEA, gas diffusion layer (SGL 39BC, SGL Carbon ltd.), and current collector (Pt-coated titanium plate). The performance of the single cell was measured using a computer-controlled power supply system (Boyaz Energy Co.) at 80 C under a DI water flow rate of 3 mL min1.
Results and discussion Fig. 1 shows the XRD patterns of pure IrO2, pure ATO, and power mixtures of IrO2 and ATO (Ir/A-70, Ir/A-50, Ir/A-30)
Intensity (a.u.)
Ir/A-70 Ir/A-50 Ir/A-30 ATO
20
was dropped on the GC electrode. After being dried in a 50 C oven for 10 min, the loading amount of the catalyst deposited on the GC electrode was ~200 mg cm2. The electrochemical performances of the samples were evaluated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). All of the electrode potentials were converted to reversible hydrogen electrode (RHE) using the Nernst equation.
Quantity Adsorbed (cm3/g STP)
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0.0
Ir/A-70 Ir/A-50 Ir/A-30 ATO
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0) Fig. 2 e N2 adsorption and desorption isotherms of pure IrO2, IrO2/ATO mixtures, and pure ATO at 77 K. prepared using a ball milling method. The sample name of IrO2/ATO mixture has defined as Ir/A-x. Ir, A, and x represented into IrO2, ATO, and ratio of IrO2/ATO, respectively (i.e. Ir/A-70 ¼ 7:3 IrO2/ATO). The pure IrOx sample showed characteristic peaks at 28.1 , 34.6 , 39.3 , 54.2 , 58.5 , and 66.1 , which corresponded to the (110), (101), (200), (211), (220), and (310) planes, respectively, of the rutile IrO2 structure (JCPDS Cards No. 43e1019, P42/mnm point group) [19]. The pure ATO sample showed characteristic peaks at 26.6 , 33.8 , 38.1 , and 51.8 corresponding to the (110), (101), (200), and (211) planes, respectively (JCPDS Cards No. 88e2348) [20]. Furthermore, using the Debye-Scherrer equation, the average particle sizes of IrO2 and ATO were determined to be ~3.2 and ~4.9 nm, respectively. In particular, as the amount of ATO increased in the powder mixtures, the peaks corresponding to the (110) plane were shifted to those corresponding to the pure ATO and the intensities of the (211) plane gradually increased [15]. This indicates that the crystal structures of IrO2 and ATO were maintained following the ball milling process. Fig. 2 shows the N2 adsorption/desorption isotherms of the as-prepared pure IrO2, ATO, Ir/A-70, Ir/A-50, and Ir/A-30. The pure IrO2 and ATO samples exhibited H4 type and IV hysteresis loops, respectively, indicating porous structures with fairly high specific surface areas of ~214 and ~131 m2 g1, respectively. According to the literature, typically, IrO2 and
Table 1 e Textural properties of pure IrO2, IrO2/ATO mixtures, and pure ATO. Catalyst IrO2 Ir/A-70 Ir/A-50 Ir/A-30 ATO
10
20
30
40
50
60
70
80
2 theta (degree) Fig. 1 e XRD patterns of pure IrO2, IrO2/ATO mixtures, and pure ATO.
a b
c
BET surface areaa (m2/g)
Pore volumeb (cm3/g)
Pore Diameterc (nm)
214 165 144 121 131
0.13 0.16 0.13 0.12 0.12
2.1 3.8 3.8 3.9 3.3
Calculated using BET equation. Pore volume evaluated from the N2 adsorption-desorption isotherms. Peak pore size determined from BJH desorption branch.
Please cite this article as: Han S-B et al., Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.109
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Fig. 3 e Pore size distribution of pure IrO2, IrO2/ATO mixtures, and pure ATO at (a) adsorption and (b) desorption branches.
ATO nanostructures had relatively lower specific surface areas than those of the samples synthesized in the present study [18,21e25]. The high specific surface areas of IrO2 and ATO as a catalyst and a support, respectively, can affect the catalytic activity because of favorable mass transfer of a reactant. With a decreasing weight ratio of IrO2, the specific
surface areas of Ir/A-70, Ir/A-50, and Ir/A-30 decreased from 165 to 121 m2 g1, and the pore volumes of the powder mixtures decreased from 0.16 to 0.12 cm3 g1 (Table 1). Based on the BJH adsorption branch, the main pore diameters of pure IrO2 and ATO were determined to be 2.1and 3.3 nm, respectively. The IrO2/ATO mixtures of Ir/A-70, Ir/A-50, and Ir/A-30 showed pore diameters of 3.8, 3.8, and 3.9 nm, respectively, which were determined based on the BJH desorption branch. Fig. 3 shows the pore size distributions of the as-prepared pure IrO2, pure ATO, and IrO2/ATO mixtures as measured using the adsorption and the desorption branches. The pure IrO2 and pure ATO were found to have pore diameters of 4.0 and 5.5 nm, respectively. In particular, the distinct peaks in the desorption branches, and not in the adsorption branches, appeared in pore diameters ranging from 3 to 4 nm, indicating a porous structure with a narrow inlet and a wide interior. Fig. 4 shows HR-TEM images of pure IrO2, pure ATO, Ir/A70, Ir/A-50, and Ir/A-30. The pure IrO2 exhibited an average particle size of ~3 nm and a high crystalline structure with an interplanar spacing of 2.9 A corresponding to the dominant {110} planes of the rutile structure (Fig. 4(a) and (f)). Furthermore, the pure ATO showed an average particle size of ~5 nm as well as a high crystalline structure with an interplanar spacing of 1.4 A corresponding to the dominant {110} planes of the rutile structure (Fig. 4(e) and (j)). Thus, the results showed that the pure IrO2 and ATO samples with fairly homogeneous nanostructures were successfully synthesized using the present synthesis method. As shown in the EDS mapping images (Fig. 5), the ATO nanoparticles as a support were surrounded by IrO2 nanoparticles as a catalyst, indicating that the IrO2 catalyst was uniformly distributed on the ATO support without serous agglomeration. Fig. 6(a) shows CVs of the pure IrO2 and IrO2/ATO mixtures measured with a sweeping rate of 50 mV s1 in 0.1 M HClO4 electrolyte solution at 25 C. The active sites of oxide catalysts for electrochemical reactions, i.e., OER, are typically proportional to the value of voltammetric charge (q), which can be measured by integrating the cyclic voltammogram [26e28]. The q values for the oxidation and the reduction reactions
Fig. 4 e HR-TEM images of (a,f) pure IrO2, (b,g) Ir/A-30, (c,h) Ir/A-50, (d,i) Ir/A-70, and (e,j) pure ATO. Please cite this article as: Han S-B et al., Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.109
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Fig. 5 e EDS images of Ir-L, SneK, and SbeK series of the IrO2/ATO mixtures.
were determined by integrating the graphs with regard to the positive and the negative directions, respectively, based on a current density of 0 mA cm2. The q values of pure IrO2, Ir/A70, Ir/A-50, and Ir/A-30 for OER were 28.3, 36.1, 22.1, and 10.9 mC cm2, respectively. The q values of pure IrO2, Ir/A-70, Ir/A-50, and Ir/A-30 for HER were 28.5, 36.2, 22.3, and 11.2 mC cm2, respectively. The total q values of pure IrO2, Ir/ A-70, Ir/A-50, and Ir/A-30 for OER were 56.7, 72.2, 44.2, and 22.1 mC cm2, respectively (Table 2). As mentioned previously, the as-prepared IrO2 showed a specific surface area (~214 m2 g1) four times higher than that of a commercial IrO2 (~50 m2 g1). Compared to the q values, among these samples, Ir/A-70 exhibited the highest q value, i.e., the highest active site for OER. Thus, Ir/A-70 with highest active site is expected to have the best electrocatalytic activity for OER. The points of inflexion were observed through the first derivative curves in the HER (Fig. 6(b)). The currents for hydrogen underpotential deposition (H-UPD) in all of the samples appeared at ~0.23 V vs. RHE. For the pure IrO2, Ir/A-70 and Ir/A-50, the electrochemical reduction reactions of Ir2þ/Ir3þ and Ir3þ/Ir4þ occurred at 0.68 and 0.83 V vs. RHE, respectively. By contrast, Ir/A-30 showed electrochemical reduction reactions of Ir2þ/Ir3þ and Ir3þ/Ir4þ at 0.65 and 0.83 V vs. RHE, respectively. The lower reduction potentials of Ir/A-30 correspond to the increased activation energies for the reduction reactions. Fig. 7 shows LSVs of the pure IrO2 and IrO2/ATO mixtures measured in a potential range from 1.0 to 1.6 V vs. RHE with a sweeping rate of 5 mV s1 in 0.1 M HClO4 electrolyte solution at 25 C. It has been reported that the oxygen evolution reaction theoretically commences at 1.23 V vs. RHE. However, the onset potential of the samples for the OER was found to be
Fig. 6 e (a) Cyclic voltammograms (CVs) and (b) first derivative curves for pure IrO2 and IrO2/ATO mixtures in 0.1 M HClO4 solution at sweeping rate of 50 mV s¡1.
Please cite this article as: Han S-B et al., Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.109
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Table 2 e Electrochemical properties of pure IrO2, IrO2/ATO mixtures, and pure ATO. CVa
Catalyst 2
IrO2 Ir/A-70 Ir/A-50 Ir/A-30 a b
LSVb 2
2
OER mC/cm )
HER (mC/cm )
Total (mC/cm )
Overpotential (V)
28.3 36.1 22.1 10.9
28.5 36.2 22.3 11.2
56.7 72.2 44.4 22.1
0.283 0.256 0.299 0.295
[email protected] V (A/cm2)
I/[email protected] V (mA/mg)
0.799 0.838 0.789 0.578
80 120 158 193
Positive area (OER) and negative area (HER) were calculated by integrating the areas divided at 0 mA/cm2 as y-axis. Overpotential was calculated by difference value from 1.23 V at 10 mA/cm2.
Fig. 7 e Comparison of LSV curves for pure IrO2 and IrO2/ ATO mixtures in 1 M HClO4 electrolyte solution.
~1.4 V, due to the loss of energy (or latent heat) required for the phase transition of H2O [11]. Furthermore, the overpotentials of pure IrO2, Ir/A-70, Ir/A-50, and Ir/A-30 at a current density of 10 mA cm2 were 283, 256, 299, and 295 mV, respectively (Table 2). Fig. 8(a) shows polarization curves of the single cells with pure IrO2 and IrO2/ATO mixtures as anode catalysts and a commercial Pt/C as a cathode catalyst. The current densities of pure Ir/A-70, Ir/A-50, and Ir/A-30 at 1.6 V were 0.799, 0.838, 0.789, and 0.578 A cm2, respectively (Table 2). Generally, the utilization amount of IrO2 catalyst for OER is a crucial consideration in terms of cost of the MEA for PEM water electrolysis. Notably, compared to pure IrO2, Ir/A-70 and Ir/ A-30 showed significantly improved OER performances, despite the fact that the amount of IrO2 was considerably reduced by up to 50 wt%. The superior OER properties of Ir/ A-70 and Ir/A-50 may be attributed to the increased electrochemical active sites, as confirmed by the CV (Fig. 6(a)). The polarization curves of the single cells were normalized with the amount of IrO2 utilized at the cathode (Fig. 8(b)). The normalized current densities of pure IrO2, Ir/A-70, Ir/A50, and Ir/A-30 at 1.6 V were 80, 120, 158, and 193 mA mg2, respectively. As the amounts of ATO increased, the catalytic efficiencies of the catalysts were found to increase. The Ir/A30 catalyst showed the highest normalized current density and the lowest current density at 1.6 V, which were attributed to the absolute deficiency of the IrO2 catalyst in Ir/A-30 mixture.
Fig. 8 e (a) Polarization curves and (b) normalized curves during PEM water electrolysis using pure IrO2 and IrO2/ ATO mixtures (cathode; 20 wt% Pt/C, temperature; 80 C, and electrolyte; Nafion 117).
Conclusions In summary, porous iridium oxide and antimony-doped tin oxide nanostructures with high specific surface areas were synthesized through the Adams fusion method. Compared to pure IrO2, the IrO2/ATO electrodes showed superior OER properties due to the increased electrochemical active sites. The mesoporous nanostructure and high specific surface areas achieved when using IrO2 and ATO as a catalyst and a support, respectively, can affect the catalytic activity in the
Please cite this article as: Han S-B et al., Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.109
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IrO2/ATO electrodes due to the favorable mass transfer of a reactant. [13]
Acknowledgments This research was supported by the Hydrogen Energy Innovation Technology Development Program of the National Research Foundation of Korea (NRF) funded by the Korean government (Ministry of Science and ICT(MSIT)) (No. 2019M3E6A1104186).
[14]
[15]
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Please cite this article as: Han S-B et al., Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.109