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Article

Effect of Electronic Conductivities of Iridium Oxide/Doped SnO2 Oxygen-Evolving Catalysts on the Polarization Properties in Proton Exchange Membrane Water Electrolysis

by
Hideaki Ohno
1,
Shinji Nohara
2,3,
Katsuyoshi Kakinuma
3,
Makoto Uchida
3 and
Hiroyuki Uchida
2,3,*
1
Special Doctoral Program for Green Energy Conversion Science and Technology, Integrated Graduate School of Medicine, Engineering and Agricultural Science, University of Yamanashi, 4 Takeda, Kofu 400-8510, Japan
2
Clean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu 400-8510, Japan
3
Fuel Cell Nanomaterials Center, University of Yamanashi, 6-43 Miyamae, Kofu 400-0021, Japan
*
Author to whom correspondence should be addressed.
Catalysts 2019, 9(1), 74; https://doi.org/10.3390/catal9010074
Submission received: 14 December 2018 / Revised: 7 January 2019 / Accepted: 8 January 2019 / Published: 11 January 2019
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Abstract

:
We have developed IrOx/M-SnO2 (M = Nb, Ta, and Sb) anode catalysts, IrOx nanoparticles uniformly dispersed on M-SnO2 supports with fused-aggregate structures, which make it possible to evolve oxygen efficiently, even with a reduced amount of noble metal (Ir) in proton exchange membrane water electrolysis. Polarization properties of IrOx/M-SnO2 catalysts for the oxygen evolution reaction (OER) were examined at 80 °C in both 0.1 M HClO4 solution (half cell) and a single cell with a Nafion® membrane (thickness = 50 μm). While all catalysts exhibited similar OER activities in the half cell, the cell potential (Ecell) of the single cell was found to decrease with the increasing apparent conductivities (σapp, catalyst) of these catalysts: an Ecell of 1.61 V (voltage efficiency of 92%) at 1 A cm−2 was achieved in a single cell by the use of an IrOx/Sb-SnO2 anode (highest σapp, catalyst) with a low Ir-metal loading of 0.11 mg cm−2 and Pt supported on graphitized carbon black (Pt/GCB) as the cathode with 0.35 mg cm−2 of Pt loading. In addition to the reduction of the ohmic loss in the anode catalyst layer, the increased electronic conductivity contributed to decreasing the OER overpotential due to the effective utilization of the IrOx nanocatalysts on the M-SnO2 supports, which is an essential factor in improving the performance with low noble metal loadings.

1. Introduction

Proton exchange membrane water electrolysis (PEMWE) is an attractive method to produce high purity hydrogen with high energy conversion efficiency, even at high current densities, together with easy maintenance, start-up, and shut-down [1,2,3,4]. Such superlative characteristics make PEMWE suitable for leveling of the large fluctuations of renewable energy sources when used in combination with stationary fuel cells. Conventional PEMWE cells, however, are costly because large amounts of noble metals are used as the electrocatalysts, e.g., (Ir + Pt) black at the oxygen-evolving anode [≥2 mg(Ir + Pt) cm−2] and Pt black at the hydrogen-evolving cathode [≥2 mg(Pt) cm−2] to maintain high conversion efficiencies with long lifetimes [2,5,6,7].
Iridium-based anodes have been employed so far, in spite of the high cost and limited availability of Ir, because they have exhibited relatively high activities and high stabilities for the oxygen evolution reaction (OER) [8,9,10]. It is essential to develop new anode catalysts that utilize Ir more effectively, working toward much higher mass activity (MA, current per mass of noble metal) for the OER, as well as high durability, while clarifying the reaction mechanisms [11,12]. In order to increase the MA, iridium or iridium oxide (IrOx) nanoparticles have been mixed or dispersed on various supports such as metal carbides [13,14,15] and oxides [16,17,18,19,20,21,22]. Considering the stability at the high oxygen-evolving potentials in strong acidic media and the need for high electronic conductivity, doped tin oxides have been reported as promising candidates as support materials [23,24]. Indeed, thin films and bulk powders of SnO2 doped with Sb, Nb, Ta, In, and F have exhibited electronic conductivities ≥0.1 S cm−1, which are sufficiently high for consideration as catalyst supports [25,26]. It has been reported that the cell potentials (Ecell) of PEMWE single cells with IrOx supported on SnO2 anodes reached values ≤1.65 V (≥ 90% voltage efficiency) at 1 A cm−2 with moderate Ir-metal loadings of 0.75 to 1 mg(Ir) cm−2 [17,18,27,28]. However, the polarization performances of such catalysts are still not sufficient in the catalyst layers of single cells for the further reduction of the Ir-loading down to 1/10 of those in conventional cells, i.e., target values of ≤0.2 mg(Ir) cm−2. One of the reasons for this is the large contact resistance between SnO2 particles, even though the bulk electronic conductivity of the doped SnO2 itself is high.
Recently, Kakinuma et al. synthesized several M-doped SnO2 (M = Nb, Ta, and Sb) materials with fused-aggregate network structures as corrosion-resistant cathode catalyst supports for polymer electrolyte fuel cells [29,30,31]. Unique advantages of these supports are their enhanced electronic conductivity and high gas diffusion rate. Onto such M-SnO2 supports, we succeeded in dispersing IrOx nanoparticles as novel anode catalysts for PEMWE. It was found that an IrOx/Ta-SnO2 catalyst exhibited an apparent MA of 15 A mg(Ir)−1 for the OER in 0.1 M HClO4 solution at 1.5 V (iR-free) vs. RHE and 80 °C, which suggests the possibility of reducing the loading of Ir in an anode catalyst to a level as low as 0.1 mg(Ir) cm−2 at a voltage efficiency of 90% (Ecell = 1.65 V) operated at 1 A cm−2, i.e., the anode potential of 1.5 V, cathode potential of −0.05 V, and the ohmic loss of the PEM of 0.10 V [32].
In the present research, we examined the polarization properties of a series of IrOx/M-SnO2 (M = Nb, Ta, and Sb) catalysts for the OER at 80 °C in both 0.1 M HClO4 solution (half cell) and a single cell with a Nafion® membrane (thickness = 50 μm). We, for the first time, found that the Ecell of the single cell decreased with the increasing apparent conductivities (σapp, catalyst) of these catalysts, whereas they exhibited similar OER activities in the half cell test. The highest performance, Ecell of 1.61 V (voltage efficiency = 92%) at 1 A cm−2 was obtained in a single cell with total noble metal loading of 0.46 mg(Ir + Pt) cm−2, in which the IrOx/Sb-SnO2 anode catalyst (highest σapp, catalyst) contribute greatly.

2. Results and Discussion

2.1. Physical Properties of IrOx/M-SnO2 Catalysts

Figure 1 shows a transmission electron microscopic (TEM) image of IrOx particles with particle size distribution, which were dispersed on Sb-SnO2 with fused-aggregate network structures (IrOx/Sb-SnO2). TEM images and particle size distribution histograms for IrOx/Nb-SnO2 and IrOx/Ta-SnO2, cited from our previous work [32], are also shown for comparison in Figure S1 in the Supplementary Materials. IrOx nanoparticles of 1 to 3 nm in diameter were found to be dispersed uniformly on the oxide supports. The average sizes and the standard deviations of the IrOx nanoparticles were 2.0 ± 0.3, 2.2 ± 0.3, and 2.0 ± 0.4 nm for the IrOx/Nb-SnO2, IrOx/Ta-SnO2, and IrOx/Sb-SnO2 catalysts, respectively. For a conventional catalyst employed as a reference (mixture of commercial IrO2 and Pt black, 1:1 mass ratio), scanning electron microscopic (SEM) and TEM images of IrO2 and Pt particles are shown in Figure S2. The average particle size of commercial IrO2 was 25 nm.
We also characterized these catalysts by BET surface area (Brunauer-Emmett-Teller adsorption method) of the M-SnO2 supports (SSnO2), the iridium loadings, the percentage of Ir4+ (IrO2) in IrOx evaluated by X-ray photoelectron spectroscopy (XPS), the amounts of M-SnO2 supports, the apparent electrical conductivities of the M-SnO2 supports (σapp, support) and IrOx-dispersed catalysts (σapp, catalyst) (see Materials and Methods, Figure S3 and Appendix S1). These results are summarized in Table 1. While Sb-SnO2 exhibited a somewhat larger SSnO2 value, similar amounts of iridium metal were loaded with similar percentages of Ir4+ on all three catalysts. Marked differences among synthesized IrOx/M-SnO2 catalysts are seen between σapp, support and σapp, catalyst values. The Sb-SnO2 support exhibited the highest σapp, support among the supports examined, i.e., three orders of magnitude higher than that of Nb-SnO2. The σapp, support values of all doped-SnO2 increased by ca. two orders of magnitude by dispersing IrOx on their surface. In particular, the σapp, catalyst of the IrOx/Sb-SnO2 catalyst was the highest value of 8.1 × 10−1 S cm−1. As reported previously for Pt/Nb-SnO2 [33] and IrOx/M-SnO2 (M = Nb and Ta) [32], such an increase in the conductivity for IrOx/Sb-SnO2 is ascribed to the shrinkage of the depletion layer of the SnO2 support particles [33]. Thus, we successfully synthesized IrOx/M-SnO2 catalysts with similar microstructures but with a range of different of σapp, catalyst values.
Since the thermodynamically stable species at OER potentials in acidic media is IrO2 (rutile) [34], the surface and/or interior of the IrOx particles on M-SnO2 can be converted to IrO2 during steady-state OER operation. Here, we compare the values of specific surface area of IrO2 (SIrO2) for commercial IrO2 powder and IrOx/M-SnO2. Assuming spherical particles of commercial IrO2 powder with 25 nm diameter based on SEM and TEM images in Figure S2, the value of SIrO2 was calculated to be 21 m2 g(IrO2)−1 or 24 m2 g(Ir)−1. In contrast, as shown in the TEM images of Figure 1 and Figure S1, the average size of the IrOx particles dispersed on the M-SnO2 support was ca. 2 nm for the as-prepared catalyst, in which the Ir4+ (IrO2) percentage was ca. 20% (Table 1), as analyzed by XPS. Then, we calculated the surface coverage of IrO2 on the particles by estimating the ratio of surface atoms to the total number of atoms (Nsurface/Ntotal), assuming that the face-centered cubic (fcc) Ir particles have an ideal cubo-octahedral shape. The calculation method [35,36] is shown in Appendix S2. The value of Nsurface/Ntotal for a 2.0 nm particle was calculated to be 52%. For the case of IrOx/Nb-SnO2 as an example, the value of 16% Ir4+ can be rationally explained if 31% of the surface atoms (=16/52) were oxidized to IrO2 in the as-prepared catalyst. If all of the surface atoms were oxidized to IrO2 with an Ir metal core during the OER, the initial particle size of 2 nm would be nearly unchanged. Thus, the value of SIrO2 (on an Ir metal core) was 133 m2 g(Ir)−1. On the other hand, if all Ir atoms in the particle were oxidized to IrO2 during the OER, the particle size could increase from 2.0 nm to 2.6 nm while maintaining a constant Ntotal, resulting in a value of 102 m2 g(Ir)−1. Thus, we are able to estimate the increase in SIrO2 of the IrOx/Nb-SnO2 by a factor of 4.3 (=102/24) to 5.5 (=133/24), compared with that of the commercial IrO2. Values similarly calculated for IrOx/Ta-SnO2 and IrOx/Sb-SnO2 are summarized in Table S1.

2.2. Oxygen Evolution Activities of IrOx/M-SnO2 Catalysts in Electrolyte Solution

Figure 2a shows the iR-free polarization curves for the OER on IrOx/M-SnO2 and conventional catalysts in air-saturated 0.1 M HClO4 solution at 80 °C. The current is shown as the apparent MA, i.e., current value per mass of Ir (or Ir + Pt for the conventional catalyst) loaded on the electrode. In order to remove oxygen gas bubbles effectively from the electrode surface, the flow rate of the electrolyte solution was adjusted at 160 cm s−1 [32]. The polarization curves for IrOx/Nb-SnO2 and IrOx/Ta-SnO2 are taken from our previous work [32]. It should be noted that a small error in the iR subtraction (ca. 10 mV at 1 A cm−2) has been corrected. These IrOx/M-SnO2 catalysts showed onset potentials for the OER from 1.35 to 1.40 V, which was similar to that for the conventional catalyst. Clearly, the MAs of the IrOx/M-SnO2 catalysts were considerably higher than that of the conventional catalyst. The values of apparent MA exceeding 10 A mg(Ir)−1 for IrOx/Nb-SnO2, IrOx/Ta-SnO2, and IrOx/Sb-SnO2 at 1.5 V were 28, 36, and 27 times larger, respectively, than that of the conventional one.
Because such enhancement factors of the MAs are much larger than those for SIrO2 described above (4.0 to 5.5 times, see Table S1), we examined the Tafel plots for the OER at IrOx/M-SnO2 and conventional catalysts, as shown in Figure 2b. Linear relationships are observed between the logarithm of MA and the iR-free potential (E) at E <1.43 V. The Tafel slope for the conventional catalyst (63 mV) was close to the commonly reported value (60 mV) for IrO2 electrodes in acidic solution [28,38]. In contrast, the values of Tafel slopes for IrOx/M-SnO2 catalysts ranged from 46 mV (IrOx/Ta-SnO2) to 52 mV (IrOx/Sb-SnO2). The existence of such low Tafel slopes, in comparison with that of bulk IrO2, implies that the OER rates on the IrOx/M-SnO2 catalysts might be promoted by an interaction between the IrOx nanoparticles and the doped SnO2 supports [28,32,39,40]. It has been also reported that an IrOx shell on an Ir core exhibited higher OER activity than IrOx [41,42]. Hence, the enhanced MAs of IrOx/M-SnO2 might be ascribed not only to a significant increase in the active surface area, by the use of IrOx nanoparticles, but also their interaction with the oxide supports.

2.3. Oxygen Evolution Activities of IrOx/M-SnO2 Catalysts in a Single Cell

We prepared catalyst-coated membranes (CCMs) with low noble metal loadings by the use of the IrOx/M-SnO2 catalysts with 0.11 mg(Ir) cm−2 at the anode and a commercial Pt/GCB (Pt supported on graphitized carbon black) with 0.35 ± 0.02 mg(Pt) cm−2 at the cathode. A conventional anode catalyst (IrO2 + Pt black, described above) with 2.66 mg(Ir + Pt) cm−2 and a Pt black cathode catalyst with 2.01 mg(Pt) cm−2 were employed in a reference CCM. The current-potential (I-E) curves of single cells operated at 80 °C are shown in Figure 3.
The performances of the cells with three kinds of IrOx/M-SnO2 anodes were found to be enhanced in the order: IrOx/Nb-SnO2 < IrOx/Ta-SnO2 << IrOx/Sb-SnO2. For example, as shown in Table 2, the Ecell at 1 A cm−2 decreased from 1.91 V for IrOx/Nb-SnO2 cell to 1.61 V for the IrOx/Sb-SnO2 cell. The latter value was somewhat larger than that of the reference (conventional) cell (1.55 V). It is noteworthy that the initial cathode performance of Pt supported on high-surface-area carbon (Pt/C) was comparable to that of Pt black, even though Pt black has still been predominantly used in practical PEMWEs in order to ensure a long lifetime of the MEA [2]. In order to mitigate the corrosion of the carbon support, we used Pt supported on GCB in place of high-surface-area carbon. In any case, we consider that the increase in the overvoltage of our cell compared with that of the conventional cell can be ascribed predominantly to the anode catalyst with reduced amount of noble metal (<1/20). As shown in Figure S4, the values of MA based on mass of Ir for the IrOx/Sb-SnO2 catalyst at Ecell >1.45 V were considerably larger than that of the conventional cell. Interestingly, the Ecell of 1.61 V for the IrOx/Sb-SnO2 cell corresponds to a voltage efficiency of 92%, which is the highest performance at the significantly low Ir loading of 0.11 mg(Ir) cm−2 at the anode reported so far [28,43,44,45].
Next, we discuss the essential parameters necessary to improve the anode performance. Referring to the properties of IrOx/M-SnO2 catalysts in Table 1, the only marked differences are seen for the values of σapp, catalyst (or σapp, support). The ohmic resistances of the cells (Rohm, cell, obs) measured at 1 kHz during the operation are shown in Table 2: the Rohm, cell, obs values ranged from 75 to 258 mΩ cm2.
To start, we calculated values of Rohm, cell, calc for comparison with the observed values. First, we estimated Rohm, anode of the anode catalyst layers (CLs) as follows. The thickness of the IrOx/Sb-SnO2 CL was ca. 10 μm, as observed by scanning ion microscopy (SIM; see Figure S5). Since we prepared all CLs in the same manner, we assumed the identical thickness for the IrOx/Ta-SnO2 and IrOx/Nb-SnO2 CLs. Assuming the porosity of the CLs to be 50%, we calculated their Rohm values based on their σapp, catalyst values. The values of Rohm, anode thus calculated for IrOx/Sb-SnO2, IrOx/Ta-SnO2, and IrOx/Nb-SnO2 were 3, 68, and 1333 mΩ cm2, respectively. Second, for the Nafion® electrolyte membrane with the thickness of 50 μm, we adopted the Rohm, Nafion to be 50 mΩ cm2. The Rohm, cell of the conventional cell in Table 2 was just 75 mΩ cm2, which is assumed to include Rohm, anode (IrO2 + Pt black: the electronic conductivity of IrO2 powder is very high [37]) and Rohm, cathode (Pt black), together with contact resistances with the gas diffusion layers (Pt/Ti mesh and carbon paper, see Materials and Methods). This value of Rohm, cell agrees precisely with those of polymer electrolyte fuel cells (PEFCs) with Nafion® membrane of the identical thickness and Pt/C catalysts for the anode and cathode [46,47,48]. Thus, by adding the Rohm, anode of IrOx/M-SnO2 to 75 mΩ cm2 stated above, we calculated the Rohm, cell, calc values to be 78, 143, and 1408 mΩ cm2, for the cells with IrOx/Sb-SnO2, IrOx/Ta-SnO2, and IrOx/Nb-SnO2, respectively. The former two values are relatively consistent with those of Rohm, cell, obs. However, a large discrepancy is seen between Rohm, cell, obs and Rohm, cell, calc for the cell with IrOx/Nb-SnO2. One of the possible reasons is that σapp, catalyst was measured in ambient air (low humidity) at room temperature, while Rohm, cell, obs was measured during operation with the anode in pure water at 80 °C. It has been shown that the electronic conductivities of SnO2-based materials increase with humidity [49,50]. Water molecules adsorbed on the SnO2 surface can act as electron donors, resulting in an increase in the carrier concentration near the surface. Such a tendency was shown to be more marked for SnO2 samples with lower electronic conductivity [49,50]. Thus, it can be easily understood that the value of Rohm, cell, obs of IrOx/Nb-SnO2 (in pure water at 80 °C) could be much smaller than that of Rohm, cell, calc. Taking into account such an effect of water on the electronic conductivity of the M-SnO2, it is appropriate to employ Rohm, cell, obs as a measure of the apparent resistance of the anode catalyst layer, rather than Rohm, cell, calc based on σapp, catalyst (measured in air).
It is clearly seen in Figure 3 and Table 2 that Ecell decreased with decreasing Rohm, cell, obs. However, this is not simply due to the reduction of the ohmic (iR) loss. For example, the reduction of the iR loss at 1 A cm−2 is only ca. 0.08 V by replacing the IrOx/Ta-SnO2 anode catalyst with IrOx/Sb-SnO2, but the reduction of the Ecell in such a case was as large as 0.23 V. On the other hand, the OER activities (MA values or Tafel slopes) of the three IrOx/M-SnO2 catalysts measured in 0.1 M HClO4 solution in the previous section can be regarded as being at a similar level.
This interesting phenomenon can be reasonably explained as follows. As illustrated in Figure 4, for the measurement of the OER activities in 0.1 M HClO4 electrolyte solution in the channel flow cell (half cell), we dispersed IrOx/M-SnO2 CLs uniformly on the Au substrate with the thickness corresponding to a ca. two-monolayer height of M-SnO2 support particles (ca. 60 nm), intending that all catalyst particles can be in contact with the electrolyte solution. Therefore, it is expected that all of the IrOx nanocatalyst particles are able to function without any influence of the small electronic (ohmic) resistances of such thin CLs. In contrast, for the measurement of single cell (MEA) performance, the thickness of the anode CL was 10 μm (ca. 170 times thicker than that in the half cell). Consequently, electrons generated at the IrOx nanoparticles in the OER must be transported in the CL to the current collector (Pt/Ti), even though protons can be effectively supplied to the IrOx surface through the electrolyte binder (ionomer) network. Hence, the higher the σapp, catalyst value (lower Rohm, cell, obs) is, the lower the OER overvoltage will be in the single cell, due to an effective utilization of the IrOx nanocatalyst particles on the M-SnO2 support.
As is clear from Figure 4b, other essential factors are the transport rates of protons and oxygen in the ionomer coated on the catalysts, in addition to the O2 gas diffusion rate in the CL. Similar to the case of CLs in PEFC [51], it is very important to control the microstructure of the CLs, i.e., thickness of the ionomer (volume ratio of ionomer to support, I/S), primary and secondary pore volumes, etc. While an effect of I/S on the performance of IrO2/TiO2 anode has been reported recently [52], more comprehensive research is necessary to optimize the single cell performance toward the near-ideal value evaluated in the half cell, together with high durability. Durability testing of single cells with IrOx/Sb-SnO2 anode catalyst is in progress in our laboratory.

3. Materials and Methods

3.1. Preparation and Characterization of IrOx/M-SnO2 Catalysts

The IrOx/M-SnO2 catalysts were prepared in the similar manner described in our previous paper [32]. Briefly, Sn0.96Nb0.04O2−δ, Sn0.975Ta0.025O2−δ, and Sn0.95Sb0.05O2−δ (projected composition, where δ is the mole fraction of oxygen deficiencies) with the fused-aggregate network structure were synthesized by the flame pyrolysis method [29]. The amount of each dopant (Nb, Ta, and Sb) was chosen to provide the highest electronic conductivity in SnO2 [29,30,31]. The surface areas of the doped SnO2 supports were measured by the BET adsorption method (BELSORP-mini, Nippon BEL Co., Osaka, Japan). IrOx nanoparticles were uniformly dispersed on the doped SnO2 supports by the colloidal method. The amounts of iridium (Ir0 + Ir4+; excluding the amount of oxygen in IrOx) loaded on the supports were quantitatively analyzed by use of an inductively-coupled plasma mass analyzer (ICP-MS; 7500CX, Agilent Technologies Inc., Tokyo, Japan) after dissolving the IrOx completely by the alkaline carbonate-fusion method.
The IrOx/M-SnO2 catalysts were observed by TEM (H-9500, operated at 200 kV, Hitachi High-Technologies Co., Tokyo, Japan). The average diameter and size distributions of the loaded IrOx particles were estimated from ca. 300 particles in more than six TEM images with 150 × 150 nm areas. To estimate the content of Ir4+, the electronic states of iridium in the IrOx/M-SnO2 were characterized by XPS (JPS-9010, JEOL Co., Ltd., Tokyo, Japan) with Mg-Kα radiation (see Figure S3). The apparent electrical conductivities of the M-SnO2 supports and IrOx/M-SnO2 catalysts were measured by the same method described in a previous paper [33]. The conventional catalyst (IrO2 and Pt black) was observed by SEM (SU9000, operated at 30 kV, Hitachi High-Technologies Co., Tokyo, Japan) and TEM.

3.2. Evaluation of OER Activities of Catalysts in Electrolyte Solution

The polarization properties of the IrOx/M-SnO2 catalysts were examined by a channel flow electrode cell technique [32]. The electrolyte solution used was 0.1 M HClO4, which was purified in advance by conventional pre-electrolysis [53]. The working electrode consisted of Nafion®-coated IrOx/M-SnO2 particles uniformly dispersed on an Au substrate with a geometric area of 0.04 cm2. The amount of the Ir catalyst loaded was maintained constant at 5 µg(Ir) cm−2. The amounts of Nb-SnO2, Ta-SnO2, and Sb-SnO2 supports thus loaded on the Au substrate were 39, 43, and 40 μg cm−2, respectively, which corresponds to a ca. two-monolayer height of the M-SnO2 support particles with an average diameter of ca. 30 nm. A mixture of commercial IrO2 (Tokuriki Honten Co., Ltd., Tokyo, Japan) and Pt black (Ishifuku Metal Industry Co., Ltd., Tokyo, Japan) was used as a reference with 100 µg cm−2 (Ir + Pt; 1:1 mass ratio). All electrode potentials are referred to the reversible hydrogen electrode, RHE.
The OER activities of the IrOx/M-SnO2 catalysts were evaluated by linear sweep voltammetry (LSV) at a sweep rate of 10 mV s−1 and 80 °C. To minimize the effect of O2 bubbles, the 0.1 M HClO4 electrolyte solution was supplied to the flow channel at a constant flow rate of 160 cm s−1. To subtract iR loss from the polarization curve, the AC impedance of the electrolyte solution was measured by a frequency response analyzer (VersaSTAT 4, Princeton Applied Research, Berwyn, PA, USA) with a modulation amplitude of 10 mV in the frequency range from 10 kHz to 1 Hz.

3.3. Evaluation of Single Cell Performances

CCMs were prepared as follows. First, the anode catalyst ink was prepared by mixing the IrOx/M-SnO2 powder, water, ethanol, and Nafion® binder solution (DE521, Du Pont Co., Tokyo, Japan) as the ionomer in a ball-mill for 30 min. The cathode catalyst ink was prepared from commercial Pt/GCB (Pt 50 wt%, TEC10EA50E, Tanaka Kikinzoku Kogyo, Tokyo, Japan). The I/S was adjusted to 0.7 (dry basis) in each ink. Then, the catalyst inks were directly sprayed onto the Nafion® membrane (thickness 50 μm, NRE 212, Du Pont Co., Tokyo, Japan) by the pulse-swirl-spray technique (PSS, Nordson Co., Tokyo, Japan) to prepare the CCM with an active geometric area of 25 cm2. The CCMs were hot-pressed at 140 °C and 2.5 MPa for 3 min. The Ir loading amount for the anode CL was 0.11 mg(Ir) cm−2, and the Pt loading amount for the cathode CL was 0.35 ± 0.02 mg(Pt) cm−2. As a reference, a conventional anode catalyst (mixture of IrO2 and Pt black, 1:1 mass ratio) with 2.66 mg(Ir + Pt) cm−2 and a Pt black cathode catalyst with 2.01 mg(Pt) cm−2 were employed. The CCM was sandwiched by two gas diffusion layers (GDLs); a Pt-plated Ti mesh (Bekaert Toko Metal Fiber Co., Ltd., Ibaraki, Japan) for the anode and a carbon fiber paper with microporous layer (25BC, SGL Carbon Group Co., Ltd., Tokyo, Japan) for the cathode. The MEA thus prepared was mounted into a single cell holder (Japan Automobile Research Institute standard cell) with ribbed single serpentine flow channels.
Ultrapure water was circulated at a flow rate of 40 mL min−1 for the anode. Hydrogen gas was purged to the cathode. I-E curves were measured galvanostatically at 80 °C under steady-state conditions. The ohmic resistance of the cell was measured by a digital AC milliohmmeter (Model 3566, Tsuruga Electric, Co., Osaka, Japan) at 1 kHz during the operation.
The thickness of the anode CL was observed after preparation of a cross-sectional sample of the CCM by SIM in a focused ion beam system (FIB, FB-2200, Hitachi High-Technologies Co., Ltd., Tokyo, Japan).

4. Conclusions

The polarization performances of the IrOx/M-SnO2 (M= Nb, Ta, Sb) anode catalysts with fused-aggregate network structures were examined for the OER in both a half cell (0.1 M HClO4) and a single cell with a Nafion® membrane at 80 °C. These catalysts exhibited similar high values of MA for the OER, regardless of the values of σapp, catalyst in the half cell, whereas the Ecell decreased with decreasing Rohm, cell, obs, catalyst in the single cell tests. In addition to the reduction of the iR loss, the predominant reduction of the anodic overvoltage is ascribed to the increased effective utilization of IrOx nanocatalyst particles supported on M-SnO2 with higher σapp, catalyst. Specifically, a single cell exhibited a promising performance Ecell = 1.61 V (voltage efficiency of 92%) at 1 A cm−2 and 80 °C with the use of an IrOx/Sb-SnO2 anode (0.11 mg(Ir) cm−2) and Pt/GCB cathode (0.35 mg(Pt) cm−2).

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/9/1/74/s1, Figure S1. TEM images and particle size distribution histograms for IrOx/M-SnO2 (M = Nb, Ta, and Sb) catalysts; Figure S2. SEM and TEM images and particle size distribution histograms for a conventional catalyst; Figure S3. XP spectra of IrOx/M-SnO2 (M = Nb, Ta, and Sb) catalysts; Figure S4. I-E curves of single cells, in which the current is shown as the apparent MA per mass of iridium metal; Figure S5. SIM image of the cross-section at the anode for the CCM with IrOx/Sb-SnO2 catalyst; Appendix S1. Calculation method for the amount of the M-SnO2 supports; Appendix S2. Calculation method for specific surface areas of IrO2; Table S1. Diameters of IrO2 (dIrO2) and specific surface areas of IrO2 (SIrO2).

Author Contributions

This work was coordinated by H.U. K.K. prepared M-SnO2 supports. H.O. synthesized all IrOx catalysts dispersed on M-SnO2 supports and carried out all physical characterization (BET, ICP-MS, TEM, and XPS), electrochemical evaluation for the half-cell and single-cell tests. S.N., K.K., M.U., and H.U. conceived all methodologies of the experiments. All authors contributed equally to discussion for the results. H.O. prepared the manuscript, and H.U. revised the final version.

Funding

This work was supported by “Fundamental Research on Highly Efficient Polymer Electrolyte Water Electrolyzers with Low Noble Metal Electrocatalysts” from Grant-in-Aid no. 17H01229 for Scientific Research (A) from Japan Society for the Promotion of Science (JSPS).

Acknowledgments

The authors appreciate Donald A. Tryk (Fuel Cell Nanomaterials Center, University of Yamanashi) for his valuable advice and Guoyu Shi (Clean Energy Research Center, University of Yamanashi) for his kind help for the single-cell tests.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Catalysts 09 00074 g001 550
Figure 1. TEM image and particle size distribution histogram for IrOx/Sb-SnO2 catalyst. The average diameter and size distribution of the loaded IrOx particles were estimated from ca. 300 particles in more than six TEM images.
Figure 1. TEM image and particle size distribution histogram for IrOx/Sb-SnO2 catalyst. The average diameter and size distribution of the loaded IrOx particles were estimated from ca. 300 particles in more than six TEM images.
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Figure 2. (a) iR-free anodic polarization curves for IrOx/M-SnO2 and conventional (IrO2 + Pt black) catalysts in air-saturated 0.1 M HClO4 solution at 80 °C with a flow rate of 160 cm s−1. The current is shown as the apparent mass activity (MA) based on the mass of Ir (or Ir + Pt for the conventional catalyst) loaded on the electrode substrate. (b) Tafel plots for iR-free anodic polarization curves shown in (a). The values of Tafel slopes for IrOx/Nb-SnO2, IrOx/Ta-SnO2, IrOx/Sb-SnO2, and conventional catalysts at E < 1.43 V were 51, 46, 52, and 63 mV, respectively. Data for IrOx/Nb-SnO2 and IrOx/Ta-SnO2 are taken from [32]; a small error in the iR subtraction has been corrected.
Figure 2. (a) iR-free anodic polarization curves for IrOx/M-SnO2 and conventional (IrO2 + Pt black) catalysts in air-saturated 0.1 M HClO4 solution at 80 °C with a flow rate of 160 cm s−1. The current is shown as the apparent mass activity (MA) based on the mass of Ir (or Ir + Pt for the conventional catalyst) loaded on the electrode substrate. (b) Tafel plots for iR-free anodic polarization curves shown in (a). The values of Tafel slopes for IrOx/Nb-SnO2, IrOx/Ta-SnO2, IrOx/Sb-SnO2, and conventional catalysts at E < 1.43 V were 51, 46, 52, and 63 mV, respectively. Data for IrOx/Nb-SnO2 and IrOx/Ta-SnO2 are taken from [32]; a small error in the iR subtraction has been corrected.
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Figure 3. Steady-state I-E curves of single cells with various anodes and Pt/GCB cathode at 80 °C. Ultrapure water was supplied to the anode with a flow rate of 40 mL min−1. The cathode compartment was purged with H2.
Figure 3. Steady-state I-E curves of single cells with various anodes and Pt/GCB cathode at 80 °C. Ultrapure water was supplied to the anode with a flow rate of 40 mL min−1. The cathode compartment was purged with H2.
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Figure 4. Schematic images of the IrOx/M-SnO2 anode catalyst layer (CL) during the OER in (a) electrolyte solution (half cell) and (b) a single cell.
Figure 4. Schematic images of the IrOx/M-SnO2 anode catalyst layer (CL) during the OER in (a) electrolyte solution (half cell) and (b) a single cell.
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Table
Table 1. BET surface areas for the supports (SSnO2), Ir loadings (Ir0 + Ir4+), IrO2 percentages (corresponding to Ir4+ vs. total Ir) in IrOx nanoparticles, M-SnO2 loadings, and apparent electrical conductivities of M-SnO2 supports (σapp, support) and IrOx/M-SnO2 catalysts (σapp, catalyst).
Table 1. BET surface areas for the supports (SSnO2), Ir loadings (Ir0 + Ir4+), IrO2 percentages (corresponding to Ir4+ vs. total Ir) in IrOx nanoparticles, M-SnO2 loadings, and apparent electrical conductivities of M-SnO2 supports (σapp, support) and IrOx/M-SnO2 catalysts (σapp, catalyst).
SampleSSnO2
(m2 g−1)		Ir (Ir0 + Ir4+) Loading
(wt %)		Ir4+ (IrO2) Percentage (%)M-SnO2 Loading (wt %)σapp, support
(S cm−1)		σapp, catalyst
(S cm−1)
IrOx/Nb-SnO23011.31688.42.5 × 10−51.5 × 10−3
IrOx/Ta-SnO22510.41989.31.3 × 10−42.9 × 10−2
IrOx/Sb-SnO24011.02188.61.8 × 10−28.1 × 10−1
commercial IrO2−−−−6.4 × 101 [37]
Table
Table 2. Noble metal loadings on CCMs, ohmic resistances (Rohm, cell, obs) and cell potentials (Ecell) at 1 A cm−2 for various cells.
Table 2. Noble metal loadings on CCMs, ohmic resistances (Rohm, cell, obs) and cell potentials (Ecell) at 1 A cm−2 for various cells.
Anode CatalystAnode Loading
[mg(Ir + Pt) cm−2]		Cathode Loading
[mg(Pt) cm−2]		Rohm, cell, obs
(mΩ cm2)		Ecell @1 A cm−2
(V)
IrOx/Nb-SnO20.110.342581.91
IrOx/Ta-SnO20.110.371751.84
IrOx/Sb-SnO20.110.35 971.61
IrO2+Pt black2.662.01 751.55

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Ohno, H.; Nohara, S.; Kakinuma, K.; Uchida, M.; Uchida, H. Effect of Electronic Conductivities of Iridium Oxide/Doped SnO2 Oxygen-Evolving Catalysts on the Polarization Properties in Proton Exchange Membrane Water Electrolysis. Catalysts 2019, 9, 74. https://doi.org/10.3390/catal9010074

AMA Style

Ohno H, Nohara S, Kakinuma K, Uchida M, Uchida H. Effect of Electronic Conductivities of Iridium Oxide/Doped SnO2 Oxygen-Evolving Catalysts on the Polarization Properties in Proton Exchange Membrane Water Electrolysis. Catalysts. 2019; 9(1):74. https://doi.org/10.3390/catal9010074

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Ohno, Hideaki, Shinji Nohara, Katsuyoshi Kakinuma, Makoto Uchida, and Hiroyuki Uchida. 2019. "Effect of Electronic Conductivities of Iridium Oxide/Doped SnO2 Oxygen-Evolving Catalysts on the Polarization Properties in Proton Exchange Membrane Water Electrolysis" Catalysts 9, no. 1: 74. https://doi.org/10.3390/catal9010074

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