Nanosphere-structured composites consisting of Cs

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 9 ( 2 0 1 4 ) 1 9 1 4 e1 9 2 3

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Nanosphere-structured composites consisting of Cs-substituted phosphotungstates and antimony doped tin oxides as catalyst supports for proton exchange membrane liquid water electrolysis Gaoyang Liu a,b, Junyuan Xu a,b, Juming Jiang a,b, Bingshuang Peng a,b, Xindong Wang a,b,* a State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, 30 College Road, Beijing 100083, China b Department of Physical Chemistry, University of Science and Technology Beijing, 30 College Road, Beijing 100083, China

article info

abstract

Article history:

Proton exchange membrane liquid water electrolyser operated blow 80 C suffers from

Received 1 July 2013

insufficient catalyst activity and durability due to the slow oxygen evolution kinetics and

Received in revised form

poor stability. Aiming at enhancing oxygen electrode kinetics and stability, composite

16 October 2013

materials consisting of antimony doped tin oxide and Cs-substituted phosphotungstate

Accepted 17 November 2013

were synthesized as the support of iridium oxide and possessed functionality of mixed

Available online 19 December 2013

electronic and protonic conductivity. At 80 C under dry ambient atmosphere, the materials showed an overall conductivity of 0.33 S cm1. The supported IrO2 catalysts were

Keywords:

characterized in sulfuric acid electrolyte, showing significant enhancement of the oxygen

Proton exchange membrane water

evolution reaction (OER) activity. Electrolyser tests of the catalysts were conducted at 80 C

electrolysis

with a Nafion membrane. At an IrO2 loading of 0.75 mg cm2 and a Pt loading of

Oxygen evolution reaction

0.2 mg cm2, the cell performance of a current density of 2 A cm2 at 1.66 V was achieved.

Composite support

The cell showed good durability at 35 C under a current density of 300 mA cm2 in a period

Antimony doped tin oxides

of 464 h.

Proton conducting phosphotung-

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

states Iridium oxide

1.

Introduction

Hydrogen as an energy carrier constitutes an important part of an environmentally benign solution to the world energy crisis. Water electrolysis is one of the most practical ways to

produce pure hydrogen from renewable energy sources [1e3]. Compared with the traditional alkaline electrolysis, proton exchange membrane (PEM) water electrolysis has great potential in hydrogen production due to its higher current densities, energy efficiency and purity of the hydrogen product [4].

* Corresponding author. State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, 30 College Road, Beijing 100083, China. Tel./fax: þ86 10 62332651. E-mail address: [email protected] (X. Wang). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.11.062


Especially for the PEM water electrolyser based on Nafion membranes operated at low temperature, it has advantages for practicability, and little demanding for thermal management and the subsequent gas product compression operations [5e7]. The activity and stability of anode electrocatalysts play an important role in the process of PEM water electrolysis, as most of the overpotential that gives rise to energy losses is related to the electrochemical processes at the anode. Typically, the anode electrocatalysts are precious metal oxides, e.g. IrO2 or RuO2. In order to improve the catalytic activity of the precious metal oxide catalysts, particularly at low loadings, an effective approach is to develop supported catalysts. An obvious advantage of using a support is a higher degree of dispersion of the catalyst nanoparticles and therefore maximized surface area of the active phase [8,9]. Requirements for catalyst support materials include sufficient electronic conductivity [10], chemical and electrochemical stability [11,12] and sufficiently high specific surface area with preferable pore-size distribution [13]. From a materials science point of view, the chemical and electrochemical stability of the OER catalyst supports are the primary challenge due to the high positive anode potential and the oxygen evolution environment. High specific surface area carbon and their nanostructured analogues e.g. carbon nanotubes are no longer applicable as the OER catalyst supports [14]. The investigated materials are a few of oxides and ceramics e.g. SiCeSi [11], TiO2 [15] and SnO2 [16], which suffer from low electronic conductivity. An obvious compromise is to use the optimal mass ratio of IrO2 of 60e90 wt% in the above supported catalysts [11,16]. The IrO2 mass ratio can be further reduced to 20 wt%, when TinO2n1 [17], TaC [12], TiC [18] and antimony doped tin oxide [19,20] with high electronic conductivity are employed as the support. Among these, antimony doped tin oxide is a conducting oxide and has a wide range of applications such as water electrolysis [21], fuel cells [22], lithium-ion batteries [23,24], solar cells [25] and gas sensors [26]. In addition to the demanded electronic conductivity in the catalyst layer, the protonic conductivity is in fact of more significance. In the oxygen evolution potential range, the OER reaction only takes place at the interface where an electronic conducting phase, a proton exchanging medium and the reactant of water molecules are present [27,28]. However, no proton conductive capacity exited neither in the reported active phase nor their supports. Generally, the proton transport was established by introducing ionomeric phases such as Nafion in the catalyst layer. However, the content of ionomers in the catalyst layer should be low, as ionomers are in general a dense phase with very low gas permeability, and therefore excessive ionomers would decrease specific active area of the catalyst particles, the porosity, mass transportation as well as the electronic conductivity of the catalyst layer [29]. In addition, the protonic conductivity of Nafion could be decreased due to the loss of sulfonic acid (eHSO3) during the OER process, which results in the rapidly declining performance. Limited to the PEM water electrolysis operated blow 80 C, other available proton conducting materials are inorganic heteropoly salts, which possess features of water-insolubility

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with the acid forms in the recent development [30e32]. Among of these heteropoly salts, CsxH3xPW12O40 (hereafter referred to CsxHPA) with the Keggin structure type is super ionic proton conductor, and exhibits high protonic conductivities (>102 S cm1) at temperature ranges of 25e80 C [33,34]. Higher protonic conductivity can be obtained in fully hydration state, which can be described as Grothuss mechanism via quasi-liquid behavior [35,36]. In the present study, antimony doped tin oxide nanoparticles were first synthesized, and on which CsxHPA was introduced as proton conducting phase to synthesize composite supports with mixed electronic and protonic conductivity. Finally, supported IrO2 catalysts were prepared and evaluated with respect to the OER activity via electrochemical and PEM water electrolyser characterizations.

2.

Experimental

2.1.

Preparation of catalyst supports

Antimony doped tin oxide nanoparticles were synthesized by chemical co-precipitation method [37]. Na2SnO3 (4.76 g, Analytically Pure, Sinopharm Chemical Reagent Co. Ltd, same for other chemicals unless otherwise specified) and Sb2O3 (0.13 g) with Sb doping content of 5 mol% were firstly dissolved in a basic mixture solution with addition of sodium hydroxide (200 g L1). Sulfuric acid (30 vol%) was then added to obtain a precipitate of Sn and Sb mixture hydroxides, which were subsequently filtered, washed with distilled water until the sulfate residues completely disappeared. The precipitate was transferred to water, heated to 70 C and the pH was adjusted to 2 by adding oxalic acid (30 wt%). Then it was mixed with polyethylene glycol as a dispersion agent and dried at 80 C in a vacuum oven for 12 h. The resulting powders were finally calcinated at 600 C for 1 h to obtain antimony doped tin oxide powders and hereafter referred to as SbeSnO2. To prepare CsxHPA, phosphotungstic acid was added dropwise to a solution of Cs2CO3 with a varied value of x (1.0, 1.5, 2.0, 2.5, 3.0). The white milky colloidal suspension was stirred overnight at 25 C and then the liquid phase was evaporated at 45 C. Being dried at 80 C in a vacuum oven for 12 h, the obtained white powders were heated at 300 C in a muffle furnace for 1 h to obtain the CsxHPA powders [38]. As shown in the conductivity measurements, the CsxHPA with a value of x ¼ 1.5 had the highest protonic conductivity. In the following synthesis of composite support from SbeSnO2, Cssubstituted phosphotungstates and supported iridium oxide catalysts, only this composition of the CsxHPA was used and hereafter referred to as Cs1.5HPA. The composite support Cs1.5HPA(XX)e(SbeSnO2) was prepared by a modified method presented by Soled et al. [39]: firstly, a certain amount of the SbeSnO2 powders were added into Cs2CO3 solution, ultrasounded 1 h, and dried at 80 C. The obtained light blue powders were then calcinated at 300 C in a pre-heated muffle furnace for 30 min to get the Cse(SbeSnO2) powders. Secondly, the Cse(SbeSnO2) powders were dispersed in distilled water, mixed with phosphotungstic acid by dropwise, stirred at 25 C overnight and dried at 45 C. The obtained white blue powders were heated at 300 C

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for 1 h. The obtained composite supports typically contained 20 wt% Cs1.5HPA and 80 wt% SbeSnO2 (Sb, 5 mol%) and hereafter were referred to as Cs1.5HPAe(SbeSnO2). The composite supports containing other contents of Cs1.5HPA were referred to as Cs1.5HPA(XX)e(SbeSnO2) with XX being 10, 40 wt %, respectively.

2.2.

Preparation of supported IrO2 electrocatalysts

Modified Adams method was used to synthesize the supported IrO2 electrocatalysts [21]. The metal precursor H2IrCl6$H2O (supplied by Beijing Nonferrous Metal Research Institute), a support powder (either SbeSnO2 or Cs1.5HPA(XX)e(SbeSnO2)) and NaNO3 (Analytical, Beijing Chemical Co. Ltd) were first mixed in distilled water. The water was gradually evaporated by heating the mixture to 80 C. The resulting salt mixture was collected and placed in a pre-heated muffle furnace and calcinated at 500 C for 30 min. The fused salt-oxide mixture was then cooled down to 25 C and washed with distilled water to remove the remaining salts. The IrO2/SbeSnO2 (in mass ratio of 1:1) and IrO2/ Cs1.5HPA(XX)e(SbeSnO2) (in mass ratio of 1:1) catalysts were finally obtained by drying at 80 C in a vacuum oven for 12 h. Pure IrO2 powders were also prepared using the same method as a reference for comparison.

2.3.

Characterizations

The crystalline structures of the prepared SbeSnO2, Cs1.5HPA, Cs1.5HPAe(SbeSnO2) and IrO2/Cs1.5HPAe(SbeSnO2) were characterized by X-ray diffraction (XRD) using a Marcogroup diffractometer (MXP21 VAHF) with a Cu-Ka radiation source (l ¼ 1.54056 A) to characterize catalyst crystalline structure. The micromorphology and particle size distribution of the prepared SbeSnO2, Cs1.5HPA and Cs1.5HPAe(SbeSnO2) were studied using scanning electron microscopy (ZEISS and LEO-1530 FESEM). FT-IR spectra of Cs1.5HPA and Cs1.5HPAe(SbeSnO2) were recorded on a PerkinElmer 1710 spectrometer under ambient atmosphere. The conductivity measurement of the supports was operated as follows. The prepared powders were placed in a homemade stainless steel chamber with a PTFE inner sleeve (the conductivity cell) and pressed by two stainless steel pressure levers with a diameter of 10 mm. The pressure was firstly increased to 3 106 Pa for 60 s and then to 3 108 Pa. Measurements were performed at 25, 35 and 80 C in dry ambient atmosphere and the thickness of the pellet in the chamber was measured before and after the test by calipers. Before testing, the CsxHPA and Cs1.5HPA(XX)e(SbeSnO2) materials were kept at 300 C for 1 h in a vacuum oven to get powders without hydrates. Electrochemical impedance spectroscopy was used for the conductivity measurement with frequency in a range of 1e106 Hz and ac amplitude of 10 mV. For electrochemical evaluations, the catalyst powder was dispersed in isopropyl alcohol (Analytically Pure, Beijing Chemical Factory), into which Nafion solution (5 wt%, Dupont) and PTFE emulsion (6 wt%, Shanghai Organic Fluorine Material Research Institute) was added. The mixtures were then homogenized for 1 h in an ice ultrasonic bath to form an ink. A 200 mL ink was deposited on a tin oxide-coated

titanium plate (20 mm*10 mm*1 mm) with an effective area of 1 cm2 and dried at 70 C. Then the electrode was dipped in the sodium chloride solution for 2 h. After that, the electrode was annealed at 340 C for 30 min and then dipped in water for 2 h. For all the catalyst inks, the mass ratio of IrO2 to Nafion and PTFE was 14:2:1 and the loading of IrO2 in the working electrode was 0.75 mg cm2. The electrochemical properties of the catalysts were characterized using VMP2 electrochemical workstation (Princeton, USA) by CV, EIS, chronopotentiometry in 0.5 mol L1 H2SO4 solution at 25 C. The potential window of CV was from 0 to 1 V vs. SCE (saturated calomel electrode) using a scan rate from 2 to 300 mV s1. The EIS measurement was conducted at 1.54 V vs. SCE for the frequency between 5 mHz and 99 kHz with a sinus amplitude of 10 mV. The impedance data were modeled with ZSimpWin software. The polarization curves were acquired between 1.2 and 1.8 V vs. SCE with a scan rate of 1 mV s1. The accelerated life tests were carried out in a 3 mol L1 H2SO4 solution at 35 C, which were employed under galvanostatic electrolysis at a constant current density of 1500 mA cm2. The service life time was cumulated till the cell voltage of electrode increased to 5 V vs. SCE [8].

2.4.

Electrolyser cell test

An ink composed of the prepared OER catalysts, Nafion solution, PTFE emulsion and isopropyl alcohol was sprayed on a PTFE sheet to form a thin anode catalyst layer. After being dipped in the sodium chloride solution for 2 h and annealed at 340 C for 30 min, the anode catalyst layer was dipped in water for 2 h. An ink composed of 40 wt% Pt/C (Johnson Matthey) as cathode catalyst, Nafion solution and isopropyl alcohol was sprayed on PTFE sheet to form a thin cathode catalyst layer. The Nafion loading in the catalyst layer was 25 wt% for both sides, and the loading of IrO2 was 0.75 mg cm2 for the anode side and 0.5 mg cm2 40 wt% Pt/C for the cathode side. Pretreatment of Nafion 212 (Dupont) was accomplished by being successively treated in 5 wt% H2O2 solution (Analytically Pure, Beijing Chemical Factory), distilled water, 0.5 mol L1 H2SO4 solution and distilled water at 80 C 60 min for each step. The catalyst-coated membrane (CCM) was obtained by transferring the catalyst film from the PTFE film to the pretreated Nafion 212 by the decal method under the conditions of 135 C, 75 kg cm2 for 3 min. The performance test was performed in a PEM electrolyser under ambient pressure at 80 C. MEA (with a 1 cm2 active area) was fabricated by placing the CCM between two carbon cloths. Distilled water was fed to the anode and cathode at a flow rate of 3 mL min1. The cathode of the cell was used as both the counter and reference electrode. Polarization curves were measured in constant current mode by increasing the current density from 0 to 2 A cm2. As for the stability test under ambient pressure at 35 C, the gas diffusion layers in the anode side were replaced for tantalum coated stainless steel felts.

3.

Results and discussion

The XRD patterns of SbeSnO2, Cs1.5HPA and Cs1.5HPAe(SbeSnO2) composite support are shown in Fig. 1.


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As shown in Fig. 1, the patterns of SbeSnO2 were matched well with the tetragonal rutile SnO2 structure as indexed with the JCPDS 21-1250 file. For the sample of SbeSnO2, no visible diffraction peaks of antimony oxide were observed, however, one should take into account of the low concentration of less than 5 mol% antimony oxide in the oxide. On the other hand, Wang et al. had reported no structure changes in the SnO2 lattice when up to 15 mol% antimony was accommodated [40,41]. The XRD spectra of Cs1.5HPA were matched well with the Keggin structure H3PW12O40 as indexed with the JCPDS 00-0500654 file. However, due to the larger space volume of Csþ than that of Hþ, the Keggin structure was expanded due to the substitution of Hþ with Csþ and thus the diffraction peak at 25.4 shifted to higher angles of 26.1 . It was reported that Cs1.5HPA with Keggin structure had the strongest acid strength and could be utilized as a high proton conducting additive [36,42]. In the case of prepared Cs1.5HPAe(SbeSnO2) composite support, characteristic diffraction peaks for both Cs1.5HPA and SbeSnO2 were observed, indicating that the successful synthesis of the composite support. Fig. 2 shows the FT-IR measurement of H3PW12O40 and the prepared Cs1.5HPAe(SbeSnO2) composite support. For the H3PW12O40 and Cs1.5HPAe(SbeSnO2), four characteristics bands of the primary Keggin structure were observed during the wave number zone 700e1100 cm1. The bands could be assigned to stretching vibration of PeOa band, W]Ot band, WeObeW corner shared bond and WeOceW edges shared bond, respectively, among which the Ob and Oc represented bridging oxygen of the Keggin structure. A split in W]O band at 985 cm1 was seen which may be assigned to the interaction between [PW12O40]3 anion and Csþ cation. Whereas, no obvious split was seen in PeOa band at 800 cm1, indicating that the substituted Csþ cation did not enter the inner boundary of the heteropoly acid anion. Csþ cation was introduced as counter cation at the outer boundary of the heteropoly acid anion. It meant that the Csþ cation had no substantive effect on the Keggin structure of Cs1.5HPA and it was consistent with the result of XRD. As reported previously, stable Keggin structure was very important to maintain high protonic conductivity of Cs1.5HPA and the composite support [36,43].

The morphologies and particle sizes of SbeSnO2, Cs1.5HPA and Cs1.5HPAe(SbeSnO2) were investigated by SEM. As shown in Fig. 3, for SbeSnO2 and Cs1.5HPA, nanoparticles with a typical diameter of around 60e100 nm were shown in Fig. 3a and b, respectively. The prepared Cs1.5HPAe(SbeSnO2) composite support (Fig. 3c) was in the form of independently distributed nanosphere structure with the similar size about 100e150 nm. As for the conductivity measurement, powders without hydrates were used to avoid the influence of the hydrogenbonded H2O molecules in phosphotungstates. That was because the number of hydrogen-bonded H2O molecules was difficult to be determined, which greatly influenced the protonic conductivity [33]. As shown in Fig. 4a, the highest protonic conductivity was obtained when the value of x is 1.5 under dry ambient atmosphere at 25 C. At any temperature, the protonic conductivities decreased slightly with the value of x increased to 2.0, 2.5 and further to 3.0. It could be concluded that the cation parameters influenced both the host lattice, the protonic entities and the dynamics [35,44]. Besides, Ukshe [33,45] and Nakamura [46] pointed out that phosphotungstates immersed in liquid water was in a highly disordered quasi-liquid state with saturated hydrates, and thus higher protonic conductivity at 101 S cm1 level could be obtained compared with the value measured above. Based on the SbeSnO2 (5 mol% Sb), composite supports were synthesized by introducing different mass contents of Cs1.5HPA. Fig. 4b showed the conductivities of SbeSnO2, Cs1.5HPA and composite supports at different temperatures. For pure SbeSnO2, the measured electronic conductivity under ambient atmosphere was 0.76 S cm1 at 25 C, 0.82 S cm1 at 35 C and 0.95 S cm1 at 80 C. For pure Cs1.5HPA, the measured protonic conductivity under ambient atmosphere was around 2.8$103 S cm1 at 25 C, increased to 3.5$103 S cm1 at 35 C and further increased to 7.3$103 S cm1 at 80 C. It was a little higher than the values reported in the literature, which were found to be of up to 103 S cm1 level without humidification [35,36,47]. When the proton conducting phase Cs1.5HPA was introduced into

Fig. 1 e XRD patterns of SbeSnO2, Cs1.5HPA and Cs1.5HPAe(SbeSnO2) composite support.

Fig. 2 e FT-IR spectra of H3PW12O40 and Cs1.5HPAe(SbeSnO2) composite support.

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Fig. 3 e SEM images of (a) SbeSnO2, (b) Cs1.5HPA, (c) Cs1.5HPAe(SbeSnO2) composite support.

SbeSnO2, the composite support displayed a steady decrease in the overall conductivity as the Cs1.5HPA content increased. The trend in the overall conductivity for the composite support suggested that the overall conductivity was controlled by the protonic conductivity of Cs1.5HPA. Introduction of Cs1.5HPA into the composite support is of importance, since the protonic conductivity in the catalyst layer is always a limiting factor of performance, which will be discussed in the electrochemical test. To evaluate the catalytic activity of the prepared catalysts, the mass specific activity based on the IrO2 loading was used in the following discussion. Cyclic voltammetry in double electrode layer region was used to characterize the electrochemical behavior of prepared catalysts. Fig. 5a showed the typical CVs for IrO2 and the supported catalysts obtained with

scanning rate of 20 mV s1 in 0.5 mol L1 H2SO4 at 25 C. All the catalysts showed the typical pseudocapacitive behavior with existing two pairs of broad redox peaks, which could be attributed to the reversible oxidation and reduction on the IrO2 surface. From CV, an integration of the voltammogram under the anode current peak was made to obtain the anode charge numbers. They were considered to be associated with the number of active sites or to the specific surface area [8,48,49]. The ratio of Qa/Qc was generally used to characterize the reversibility of redox process, which was obtained by calculating the anode and cathode charge of the voltammogram between 0 and 1 V vs. SCE. According to the study of Marshall et al. [13], the voltammograms were measured at a scan rate of 300 mV s1 to obtain higher current densities and thus the

Fig. 4 e (a) Conductivities of CsxHPA with varied value of x (1, 1.5, 2, 2.5, 3) at different temperatures. (b) Conductivities of SbeSnO2, Cs1.5HPA and their composites containing varied amounts of Cs1.5HPA at different temperatures.


higher the differences on Qa/Qc ratios of all the prepared catalysts could be compared. The Qa/Qc ratios of all the prepared catalysts were listed in Table 1. Only for the catalyst with the composite support (Cs1.5HPAe(SbeSnO2)), the Qa/Qc ratio was close to 1, which confirmed that the reversible behavior of surface redox transition was best. This result could correlate with the dispersion of active phases and the conductivity of the composite support [12,15]. Fig. 5b showed the voltammetric charges for IrO2 and the supported catalysts obtained at different potential scan rate. It could be seen that the charges increased slightly with decreasing scanning rates, especially at the lower scanning rates. Ardizzone [50,51] and Trasatti [52] pointed out that the charges increases with decreasing scanning rates mainly due to the difficulties of proton exchange on the interface between active phase and electrolyte. At very high scanning rates, proton exchange took place only on more accessible “outer” active surface, but occurred on the whole active surface (both the “outer” and less accessible “inner” surfaces) at very low potential scan rates. Correspondingly, the charge related to the “outer” active surface was that directly exposed to the electrolyte, and the charge normally related to less accessible “inner” surfaces was that proton exchange took place relying on proton diffusion.

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The obtained total charge, outer charge and charge ratio of all the prepared catalysts were listed in Table 1. As for the IrO2 and SbeSnO2 supported IrO2 catalyst, an obvious increase of the total charge number, or in other words, the number of the catalytically active sites, was observed. It could be explained by the better dispersion of the iridium oxide particles on the supports. While the charge was limited by the proton diffusion at the subsurface regions and thus only slightly increase in the Qo/Qt was obtained. A more significant improvement in the catalytic activity was achieved when Cs1.5HPA was introduced into the SbeSnO2 as the support of IrO2. The better catalyst performance could be attributed to the mixed conductivity in the composite support and uniform nanosphere structure. The typical Nyquist diagrams of electrochemical impedance behavior for the three type prepared catalysts in Fig. 4c were measured at 1.54 V vs. SCE, which corresponded to the oxygen evolution region. The impendence parameters for the IrO2, IrO2/SbeSnO2 and IrO2/Cs1.5HPAe(SbeSnO2) catalysts were described in Table 1, which were obtained from fitting the experimental data using ZSimpWin software. An appropriate equivalent electrical circle (EEC) shown in the inset of Fig. 4c was selected to fit the impedance data, where Rohm was the ohmic resistance of working electrode and electrolyte

Fig. 5 e (a) Cyclic voltammograms of the three types of prepared catalysts at scan rate of 20 mV sL1. (b) Charge as functions of scan rate of prepared catalysts. (c) Nyquist diagram of three types of prepared catalysts measured at 1.18 V vs. SCE during oxygen evolution, the inset was equivalent circuit. (d) Tafel curves of the three types of prepared catalysts at scan rate of 1 mV sL1. All electrochemical test was operated in the 0.5 mol LL1 H2SO4 at 25 C.

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Table 1 e Summary of electrochemical characterization of prepared catalysts. CV

IrO2 IrO2/SbeSnO2 IrO2/Cs1.5HPAe(SbeSnO2) IrO2/Cs1.5HPA(10 wt%)e(SbeSnO2) IrO2/Cs1.5HPA(40 wt%)e(SbeSnO2)

EIS

Tafel slopes

Qa/Qca

Qt a

Qoa

Qo/Qta

Rohmb

Rctb

Qdlb

S1c

S2c

1.051 1.017 1.007 1.009 1.021

258 568 769 704 386

213 480 721 619 320

0.826 0.845 0.938 0.879 0.829

0.207 0.073 0.076 0.111 0.187

4.573 2.898 1.333 2.263 3.352

206 435 676 490 315

63 56 41 49 62

393 253 201 232 274

The anodic to cathodic charge ratio (Qa/Qc), total charge (Qt/mC cm2 mg(IrO2)1), outer charge (Qo/mC cm2 mg(IrO2)1) and charge ratio (Qo/ Qt) of all prepared catalysts calculated from the cyclic voltammograms. b Ohmic resistance (Rohm/ohm cm2), charge-transfer resistance (Rct/ohm cm2) and overall capacitance (Qd1/mF cm2). c Tafel slopes (S1 for the low current density regime/mV dec1, S2 for the high current density/mV dec1). a

between working electrode and reference electrode, Rct was the charge-transfer resistance of a faradic process occurring at the oxide/electrolyte interface. The capacitive elements were represented by Q, a constant phase element (CPE) often used to model the depressed semi-circles due to heterogeneities and surface roughness. Here, Qd1 included both the double-layer capacitance of the oxide-electrolyte interface and the pseudocapacitance of catalysts. This type of model had generally been used to interpret the impendence data of IrO2 or RuO2 where the R1Q1 circuit was assumed to be attributable to different phenomena, e.g. ionic or electronic transport phenomena [53], effect promoted by the presence of pores in the electrode coating [54] or the resistive effect of the Ti2O3/TiO2 interlayer on the titanium substrate [55]. As seen from Table 1, the Rohm of IrO2/SbeSnO2 and IrO2/ Cs1.5HPAe(SbeSnO2) electrocatalysts were approximately 0.07 ohm cm2, which was much lower than that of IrO2 electrocatalyst (0.207 ohm cm2). Ohmic voltage drop caused by Rohm might be large at high current densities, where ohmic resistivity could not be neglected. The charge-transfer resistance (Rct) in the low frequency range, which was a measure of the polarization resistance or the catalytic activity of the electrode, was directly related to the OER kinetics. The Rct was decreased immensely from 4.573 ohm cm2 for IrO2, 2.898 ohm cm2 for IrO2/SbeSnO2 and to 1.333 ohm cm2 for

Fig. 6 e Potential variation vs. time in the accelerated life test performed in a 3 mol LL1 H2SO4 solution under 1500 mA cmL2 at 35 C.

IrO2/Cs1.5HPAe(SbeSnO2) catalysts, showing the steady enhancement of the catalytic activity. Moreover, the overall capacitance (Qd1), a measure of the active surface of the catalysts [45], was found to be 206 mF cm2, 435 mF cm2 and 676 mF cm2 for the three types of catalysts. In brief, the low charge-transfer resistance and the high capacitance or active surface area for the composite supported catalyst suggested the enhanced catalytic activity for the oxygen evolution. Otherwise, as shown in the Table 1, the optimal Cs1.5HPA content in the composite support was 20 wt%, and the catalytic activity of composite supported IrO2 catalysts decreased when the Cs1.5HPA content was further increased in the composite support. The better catalyst performance could due to the mixed conductivity in the composite support because any catalytically active sites should possess both electronic and protonic conductivities and the protonic conductivity is always limiting. Steady state polarization curves of the three prepared catalysts were recorded in the potential region for the oxygen evolution reaction (1.2e1.8 V vs. SCE) as shown in Fig. 5d and corresponding Tafel slopes were listed in Table 1. A two region polarization with significantly different Tafel slopes was observed in the studied current density range for all the

Fig. 7 e Currentevoltage curves of PEM water electrolysis cells with different anode catalysts operating at 80 C with Nafion 212 membrane. The operational pressure was ambient and the cathode was made of 40 wt% Pt/C with a Pt loading of 0.2 mg cmL2.


catalysts in the electrolyte. The existence of two Tafel regions might be an indication of changes in the reaction mechanisms due to the adsorption contribution of intermediates [16]. The Tafel slope on the prepared catalysts was decreased in the order of IrO2, IrO2/SbeSnO2 and IrO2/Cs1.5HPAe(SbeSnO2). In accordance to the CV studies, EIS and steady state polarization studies discussed above, the effect of catalyst supports on the catalytic activity of IrO2 may be the followings: (1) The dispersion of iridium oxide particles on both the SbeSnO2 and composite nanosphere-structured supports increased the mass active area. (2) The IrO2 catalysts supported on SbeSnO2 as well as Cs1.5HPAe(SbeSnO2), most likely attributed to the additional improvement of the support conductivity, particularly the protonic conductivity, which could provide more electrocatalystic sites. (3) The optimal content of Cs1.5HPA in the composite supports was 20 wt%, and higher Cs1.5HPA content in relation to the SbeSnO2 resulted in increased Rohm, Rct and Tafel slopes. Briefly speaking, both the catalyst dispersion in terms of the specific surface area and the overall conductivity of catalysts were essential for the catalyst polarization performance. The accelerated life tests were used as a rapid stability evaluation for comparison of pure IrO2 and the two types of supported catalysts, and the corresponding results were shown in Fig. 6. The service life of the electrodes increased in the order: IrO2 < IrO2/SbeSnO2 < IrO2/Cs1.5HPAe(SbeSnO2) and the service life of the IrO2/Cs1.5HPAe(SbeSnO2) electrode was about 142 hours. It should be remarked that the electrodes were prepared with the same loading of IrO2 and fabrication process in order to investigate the effect of anode catalysts with different supports. The much longer service life of the IrO2/Cs1.5HPAe(SbeSnO2) electrode was primarily attributed to the introduction of the composite support. MEAs with different anode catalysts were prepared and tested in a homemade electrolyser. As seen from Fig. 7, the electrolysis cell performance operated at 80 C under ambient

Fig. 8 e Stability of PEM water electrolysis cells operating at temperature of 35 C and current density of 300 mA cmL2 under atmospheric pressure. The anode consisted of IrO2/ Cs1.5HPAe(SbeSnO2) catalyst at an IrO2 loading of 0.75 mg cmL2 and the cathode was made of 40 wt% Pt/C at a Pt loading of 0.2 mg cmL2. The membrane was Nafion 212 membrane. The inset was polarization curves at 100, 464 h.

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pressure was steadily improved in the order: IrO2 < IrO2/ SbeSnO2 < IrO2/Cs1.5HPAe(SbeSnO2). Due to the better dispersion of the iridium oxide particles on the SbeSnO2 and therefore the increased number of active sites, the performance of IrO2/SbeSnO2 was improved compared with pure of IrO2 catalyst. A more significant improvement in the performance was achieved when Cs1.5HPA was introduced into the SbeSnO2, which could be attributed to the mixed conductivity in the composite support. The terminal voltage of electrolysis cell of MEA prepared with IrO2/Cs1.5HPAe(SbeSnO2) catalyst was 1.66 V at current density of 2 A cm2. The durability of the electrolyser performance is one of the critical concerns for characterizations of materials and components. To evaluate the stability of the IrO2/ Cs1.5HPAe(SbeSnO2) catalyst, an electrolysis cell was tested under a constant current density of 300 mA cm2 at 35 C with a period of 464 h. As shown in Fig. 8, in the initial 100 h, the voltage had a sharp increase from 1.68 to 2.03 V. It could be caused by the fact that the blocking of membrane exchanging sites by some metal cations e.g. tungsten ions, iron ions and resulted in increasing the electrochemical overpotential (in the inset of Fig. 8) [56]. After stabilization period of the initial 100 hours, the cell presented reasonably good stability for 364 hours and the degradation rate of cell voltage was about 0.52 mV h1, indicating that this MEA can continuously operate for more time without sharply increase of voltage. It should be noted that no systematic optimization of the protonic conductivity of the MEA were made in the present work, which could be expected to further improve both the cell performance and durability.

4.

Conclusions

Novel catalyst composite support with functionalities of mixed protonic and electronic conductivities was synthesized from antimony doped tin oxide and proton conducting Cssubstituted phosphotungstates. The composite support was structured with nanosphere and an overall conductivity of 0.33 S cm1 with a contribution of the proton conduction was achieved at 80 C under dry ambient atmosphere. Using various types of supports including this nanospherestructured composite support, iridium oxide nanoparticle catalysts were prepared. The mass catalytic activity, as measured in sulfuric acid electrolytes, was found to be increased in the order of IrO2 < IrO2/SbeSnO2 < IrO2/ Cs1.5HPAe(SbeSnO2). Electrolyser test of the composite supported catalyst was operated at 80 C, and the terminal voltage was 1.66 V at 2 A cm2 with an IrO2 loading of 0.75 mg cm2, a Pt loading of 0.2 mg cm2 and a Nafion 212 membrane as electrolyte. At 35 C and 300 mA cm2 the cell showed good stability with a period of 464 h.

Acknowledgements Financial support for this work is acknowledged from the 973 Program of China (Project No. 2013CB934002), the 863 Program of China (Project No. 2012AA053401), the National Natural

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Science Foundation of China (Grant No. 51274028) and Natural Science Foundation of Beijing (Grant No. 2122041). [18]

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