Vibrating Powders: Electrochemical Quartz Crystal Microbalance

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C: Energy Conversion and Storage; Energy and Charge Transport 2

Vibrating Powders: Electrochemical Quartz Crystal Microbalance Study of IrO and Pt/C Catalyst Layers for Voltage Reversal Tolerant Anodes in Fuel Cells Colin E. Moore, Foroughazam Afsahi, Alan P. Young, and Elod L. Gyenge J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05652 • Publication Date (Web): 04 Sep 2019 Downloaded from pubs.acs.org on September 4, 2019

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The Journal of Physical Chemistry

Vibrating Powders: Electrochemical Quartz Crystal Microbalance Study of IrO2 and Pt/C Catalyst Layers for Voltage Reversal Tolerant Anodes in Fuel Cells

Colin E. Moore1,2, Foroughazam Afsahi2, Alan P. Young2, Előd L. Gyenge1* 1Department

of Chemical and Biological Engineering, Clean Energy Research Centre University of British Columbia, Vancouver, BC Canada 2Ballard

Power Systems, Burnaby, BC Canada

*Corresponding

author (email: [email protected])

Abstract Fundamental understanding of the interaction between Pt and IrO2 catalysts in proton exchange membrane fuel cell (PEMFC) anodes is important to mitigate degradation during cell voltage reversal events by catalyzing the oxygen evolution reaction (OER) instead of carbon corrosion and Pt dissolution. The potential dependent mass changes of untreated and different heat treated (350, 450 and 550 oC) IrO2 and IrO2+Pt/C powders were investigated in a simulated PEMFC environment using the electrochemical quartz crystal microbalance (EQCM) technique. During successive potential cycling the frequency changes associated with IrO2 were correlated with the potential dependent uptake of water and formation of iridium oxyhydroxide species; confirmed by X-ray photoelectron spectroscopy (XPS). Heat treatment of the IrO2 powders increased crystallinity which triggered changes in the frequency response during accelerated cyclic voltammetry stress tests. During anodic polarization, the untreated IrO2 exhibited a steady increase 1 ACS Paragon Plus Environment

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in frequency beginning at ca. 0.6 V vs. RHE; whereas the heat treated IrO2 first revealed a frequency decrease, then levelled off before increasing. These observations were explained based on differences in potential dependent water uptake and Ir oxyhydroxide formation as a function of IrO2 structural variations from amorphous to crystalline and Ir3+/4+ ratio. Pt/C alone when subjected to potential hold at 1.8 V and 2.0 V vs. RHE experienced significant mass decrease due mainly to carbon corrosion. The addition of IrO2 to Pt/C in the catalyst layer did not show any mass decrease during the same potential hold. The OER catalyzed by IrO2 protected Pt/C from degradation (i.e., carbon corrosion and Pt dissolution) at high potentials and created an ex situ voltage reversal tolerant anode. 1. Introduction Oxygen evolution reaction (OER) catalysts are added to Pt/C in the anode catalyst layer to protect carbon components of PEMFC anodes during transient fuel starvation conditions such as startup/shut-down.1 OER catalysts such as IrO2 or RuO2 promote the oxidation of water on the anode surface instead of the corrosion of carbon components and Pt detachment/dissolution.1,2 The mechanism of this protection is twofold. First, under voltage reversal conditions the OER catalyst lowers the overpotential of the OER, which prevents the anode potential from rising to the carbon corrosion onset potential.3–5 C + 2H2O → CO2 + 4H+ + 4e-,

E°298K = 0.21 VSHE

(1)

Second, a fast OER reaction will increase the local H+ activity while lowering the local H2O activity on the fuel cell anode surface. According to the Nernst equation these local activity changes will trigger the increase of the equilibrium potential for the carbon corrosion (equation 1).

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Thereby, the overpotential for reaction (1) is diminished at a given anode potential, which in turn lowers the carbon corrosion rate. The durability of OER catalysts have been extensively investigated mainly under water electrolyzer operating conditions.6–11 However, the conditions in a PEMFC anode during normal operation (ca. 0 V and low pH) can induce instability in these metal oxides. IrO2 in particular can form soluble Ir3+ species11 that migrate across the membrane electrode assembly and deposit on the cathode causing poisoning of the oxygen reduction reaction (ORR) catalyst (Pt/C). Typically, the suitability of OER catalysts for voltage reversal tolerant anodes (RTAs) is investigated using prototype membrane electrode assembly (MEA) single cell testing, which is time consuming and expensive to perform.12–14 Therefore, effective ex situ methods are needed to provide OER catalytic activity and durability related information in a simulated PEMFC environment that reliably translates to fuel cell anode performance with respect to voltage reversal tolerance. Furthermore, in order to validate these new ex-situ methods, it is important to study a series of OER catalysts (such as IrO2) with known properties and well-understood OER activity. Heat treatment can reduce the activity and increase the durability of IrO2 catalysts as they transition from amorphous to crystalline. This has been shown by da Silva et al. who heat treated IrO2 catalysts in a range from 100 °C to 500 °C at 100 °C intervals.10 Increased durability after heat treatment was confirmed by identical location transmission electron microscopy (ILTEM) where IrO2 heat treated at 500 °C showed no morphological changes after 500 aging cycles (1.1 to 1.6 V vs. RHE, 50 mV s-1). Similar heat treatments have been used to produce more durable but less active IrO2 and metal oxide supported IrO2 OER catalysts.15–18 To develop efficient ex situ techniques, our group investigated the dissolution of unsupported and supported IrO2 catalysts using an ex situ accelerated stress test based on potential stepping. The 3 ACS Paragon Plus Environment


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dissolution of Ir3+ from this test was compared with the in situ voltage reversal tolerance of a membrane electrode assembly fabricated using the same IrO2 catalyst.19 For the unsupported catalysts, we were able to correlate iridium dissolution (measured by inductively coupled plasma mass spectrometry (ICP-MS)) in the three electrode ex situ cell to in situ voltage reversal loss in a single cell MEA. Reversal loss was defined as the reversal time of a freshly prepared MEA minus the reversal time of an MEA after it had undergone an accelerated anode stress test (AAST). While Ir3+ dissolution can be effective in determining promising OER catalyst candidates, it has one major limitation. ICP-MS only provides the Ir3+ concentration in the bulk of the electrolyte at the end of a stress test and it does not account for Ir3+ possibly consumed at the counter electrode. To address this issue, here the EQCM technique is employed for both IrO2 and IrO2+Pt/C powder catalyst films. The EQCM uses a piezoelectric sensor (in this case a vibrating quartz crystal) where the vibration frequency is measured concurrently with potential. This vibration frequency is inversely proportional to the mass of the crystal according to equation (2). ∆𝑓 = 𝐶𝑓∆𝑚

(2)

where Δf is the observed frequency change (Hz), Δm is the mass change per unit area (µg cm-2) and Cf is the quartz crystal sensitivity factor (referred to also as the Sauerbrey constant) (Hz μg-1 cm2). The conversion of observed frequency to mass via equation (2) is strictly valid only for a rigid uniform film on the surface to the crystal. Electrochemical changes of an electrodeposited IrO2 layer were investigated using EQCM by Birss et al.20 They reported a frequency increase during the oxidation sweep at potentials greater than 0.2 V vs. SCE accompanied by a frequency decrease on the reverse reduction scan virtually without any hysteresis of the frequency profile. After ruling out possible viscoelastic effects, the observed frequency changes were mainly attributed to the potential dependent uptake and ejection 4 ACS Paragon Plus Environment

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of water and ions in the hydrous Ir oxide film during the redox shuttle between Ir3+/4+ oxyhydroxides. This is also consistent with Burke and Scannell’s explanation of the hydrous Ir oxide cyclic voltammograms.21 However, the formation of Ir oxyhydroxide was not confirmed by separate surface analytical techniques; only the presence of counter ions in the hydrous films were confirmed by XPS and ion chromatography.20 EQCM has been previously used to study electrochemical processes occurring on catalyst powder layers.22–26 Ofstad et al. prepared a Pt/C ink containing Nafion® which was then sprayed onto EQCM quartz crystals.25 They were able to identify increased Pt dissolution and lower onset potential of Pt oxide formation for Pt/C layers in the presence of chloride ions. Platinum dissolution or degradation from Pt/C is more significant than dissolution from electrodeposited Pt layers. Rice et al. calculated the mass of Pt oxide on Pt/C using single cycle asymmetric voltammetry coupled to EQCM. This work focused only on PEMFC cathode side catalysts.24 In the present work, the effect of heat treatment on potential dependent mass changes of IrO2 powders is investigated for the first time using EQCM. We also combine IrO2 with Pt/C to form a layer that mimics a voltage reversal tolerant PEMFC anode catalyst. With this layer we investigate the potential dependent frequency changes and provide insight into the processes occurring at the electrode surface in order to de-convolute the EQCM response into its constituent parts. Finally, the addition of IrO2 to Pt/C is investigated for its ability to protect against carbon corrosion at high potentials.



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2. Experimental Methods 2.1. Iridium Oxide Heat Treatment and Surface Area Analysis IrO2 (Alfa Aesar, 99%) was heat treated at three temperatures (350, 450 and 550 °C) in air using a tube furnace (Barnstead Thermolyne 21100). First the catalyst was placed in an alumina crucible. The powder was confirmed not to have any large chunks before the crucible was introduced into the tube furnace. Air was introduced to the tube using a custom gas manifold and the temperature was increased to the set-point. The heat was maintained for two hours and then allowed to decrease to 50 °C before removing the crucible. Surface area of the catalysts was determined using the Brunauer-Emmett-Teller (BET) method with a Nova 2000e surface area and pore size analyzer (Quantachrome Instruments). Table 1 presents the surface area of each heat treated catalyst. Table 1. BET Surface Area of IrO2 Untreated and Heat Treated at 350, 450 and 550 °C. IrO2

Surface area m2 g-1

Untreated

28.2

350 °C

27.4

450 °C

22.5

550 °C

19.0

2.2. Ink Preparation The IrO2 ink was prepared by sonicating (Branson 3510 ultrasonic) 4 mg of IrO2 powder in 1.2 mL 2-propanol (Alfa Aesar, 99.5%) for 1.5 hours until a well dispersed dark black ink was formed. 10 µL of this solution was drop cast onto the center the piezo-active area of an EQCM crystal (Stanford Research Systems, 5 MHz AT-cut quartz, 1.37 cm2 electroactive area, 0.4 cm2 piezo6 ACS Paragon Plus Environment

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active area, 331 μm crystal thickness, gold coated) and allowed to dry. The target crystal loading was 24 μg cm-2 electroactive area. The goal of this technique was to obtain a uniform and repeatable ink deposition. Platinum on carbon (Pt/C, Tanaka TEC10E50E, Pt 50 wt% on “high surface area carbon”, Fig. S3) was also admixed with two types of IrO2, untreated and heat treated at 550 °C. 3 mg of the Pt/C and 1 mg of the IrO2 was sonicated in 1.2 mL of 2-propanol for 1.5 hrs. The ratio of Pt metal to Ir metal was 2:1 in the ink. 3 μL of the resulting ink was drop cast onto the EQCM quartz crystal. This resulted in a total solids loading of 7.3 μg cm-2. Pt/C alone was prepared in the same way but without the addition of IrO2 which resulted in a total solids loading of 5.5 μg cm-2. Ketjen Black (EC600JD, 1270 m2 g-1, 34.0 nm primary particle radius) was also prepared into an ink in the same way. In this case, 1.5 mg of the powder was dissolved in 1.2 mL of 2-propanol. This was to maintain the same carbon loading as in the 50 wt% Pt/C whereas the total solids loading was 2.7 μg cm-2. 2.3. Quartz crystal microbalance measurements The ink drying QCM measurements and EQCM measurements were made using a QCM 200 system (Stanford Research Systems) connected to the integrated analog to digital converter of a Metrohm Autolab PGSTAT302N and recorded using Metrohm NOVA software. The EQCM measurements were carried out in a three electrode cell configuration consisting of a platinum counter electrode, reversible hydrogen electrode (RHE) and EQCM crystal secured in a Teflon holder as the working electrode. All experiments were carried out using N2 purged 0.09 M H2SO4 electrolyte prepared with double deionized water (Thermo Barnstead GenPure, 18.2 MΩ cm-1). The electrolyte concentration was chosen to mimic the pH (~ 1) of the anode catalyst layer in a PEMFC.19 7 ACS Paragon Plus Environment


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2.4. EQCM Characterization and Accelerated Potential Stress Testing Each catalyst was investigated using the following accelerated cyclic voltammetry (CV) stress test. Thirty CV cycles were carried out using three potential windows (1.2, 1.4 and 1.6 V to 0.05 vs. RHE) in 0.09 M H2SO4 at 25 °C. The potential was swept from high to low with simultaneous measurement of the frequency changes. These potential windows were chosen to approximate three PEMFC anode conditions: a gas switching stress test (0.05 to 1.2 V vs. RHE), “low” voltage reversal conditions (0.05 to 1.4 V vs. RHE) and “high” reversal conditions (0.05 to 1.6 V vs. RHE). Potential holds at 1.8 and 2.0 V vs. RHE were also carried out to investigate the accelerated degradation of the Pt/C and the effect of IrO2. 2.5. Imaging and Characterization Scanning electron microscopy (SEM) images were obtained using a TESCAN VEGA3 system. IrO2 layers drop cast on EQCM crystals were imaged both before and after the potential cycling protocol was applied. IrO2 crystal structures were analyzed as received and after heat treatment by X-ray powder diffraction (XRD) using a Bruker axs-D8 Advance system with a Cu radioactive X-ray source along with a LynxEye silicon strip detector. XRD measurements were taken at 40 kV and 40 mA. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos Analytical Axis Ultra DLD system. Scanning transmission electron microscopy (STEM) was carried out using an FEI Osiris system with a high-angle annular dark field (HAADF) STEM detector and a bright field (BF) detector. Inductively coupled plasma mass spectrometry (ICP-MS) was performed on some electrolytes to determine the concentration of dissolved metals after electrochemical testing. This was carried out by an external service, Exova (Surrey, BC).


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3. Results and Discussion 3.1. EQCM Frequency to Mass Relationship Fig. 1 presents the change in frequency during multiple ink deposition and drying steps on an EQCM crystal. Similar experiments have been previously reported.27

0

detachment of particles

20 -4000

Drying

+5 L drying time shortened

-6000

5

10

15

50

6000

40

30 40

+5 L

20

25

30

4000

20

50

crystal nearly overloaded

-8000 0

R2 = 0.9997

10

+5 L

60

b

8000

mass / g fdeposition / Hz

-2000

a

mcalculated / g

2-propanol viscous coupling

0

f / Hz

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 30

2000 10

time / min

20

30

40

50

10 60

m Inkdeposition / g

Figure 1. (a) Deposition and drying (under a heat lamp) of an ink composed of IrO2 and 2-propanol deposited on the surface of an EQCM crystal. The ink was drop cast in three 5 μL steps (at 0, 12 and 22 minutes) to observe the detachment of particles during multiple depositions. (b) Plot of the deposit frequency change after each ink drop vs. the mass of IrO2 present in the volume of ink. The second y-axis is the calculated mass change from the observed frequency change after each deposition. Fig. 1a shows the frequency response of three consecutive 5 μL deposition steps on a gold EQCM crystal. During the first deposition, the frequency decreases by ca. 930 Hz (6.5 μg) because of the viscous coupling of the ink. This is similar to immersing the electrode into an ink solution where the frequency decrease is dependent on the viscosity of the solution.28 During subsequent deposition steps, the frequency first increases indicating detachment of particles from the gold 9 ACS Paragon Plus Environment


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surface. The frequency drops again during the drying step. The mass scale in Fig. 1 (and later EQCM Figures) was calculated using a conversion factor of 7.08 ng Hz-1. The latter factor was obtained by multiplying the sensitivity factor (Cf equation (2)) with the piezo-active area (0.4 cm2). Vatankhah et al. showed that there is variation in the sensitivity factor based on Cu film thickness.29 Using the Cu deposition technique (Figs. S1 and S2, Supplemental Material), we found an integral sensitivity factor of 47.8 Hz cm2 μg-1 (vs. the theoretical Cf of 56.6 Hz μg-1 cm2 for the 5 MHz AT-cut quartz crystal). However, in order to obtain the true sensitivity factor that is relevant for the film thicknesses obtained with the various IrO2 and Pt/C catalyst powders, multiple Cu deposition calibration procedures would be needed such that to approximate the actual catalyst film thickness. Even if this would be accomplished, the sensitivity factor for a rigid film (Cu layer) will be different than the one for a looser powder film. For these reasons, we have decided to use the theoretical sensitivity factor of a standard AT-cut 5 MHz quartz crystal for mass change calculations using equation (2) (Fig. S1 and S2, Supplemental Material). The use of the 5 MHz theoretical sensitivity factor was supported by the multiple IrO2 ink deposition experiments in which a known mass was deposited on the surface of the EQCM. The sensitivity factor calculated mass change was 18.4 ± 0.6 μg while the mass calculated based on the mass of IrO2 in the ink was 17.1 μg for one 5 μL deposition; a 7.6% difference (Fig. 1b). 3.2. Heat Treated IrO2 XRD, XPS and SEM Characterization Fig. 2 shows the XRD spectra of each IrO2 catalyst. The untreated IrO2 has one identified metallic Ir phase and an amorphous phase (presumed to be IrO2). The heat treated catalysts all have identifiable Ir metal and IrO2 crystal phases. Metallic Ir peaks are much smaller in the IrO2 heat treated at 550 °C. Similar heat treatment performed by da Silva et al. determined that IrO2 heat treated up to 300 °C was similar to untreated IrO2 and remained amorphous in nature. However, 10 ACS Paragon Plus Environment

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heat treatment at 400 °C caused IrO2 to become more crystalline and at 500 °C the peaks became thinner and sharper with crystal sizes between 3.4 and 13.7 nm respectively.10 We were able to confirm these previous findings by showing that starting at 350 °C IrO2 becomes more crystalline and less amorphous in character (Fig. 2). Additionally, increased peak height and a narrowing of the peak width is observed as the heat treatment temperature was increased to 450 and 550 °C. IrO2 (101)

IrO2 (110)

IrO2 (211) IrO2 (200)

IrO2 (220)

IrO2 (310)

550°C 450°C 350°C Ir (220) Ir (111) Ir (200)

5

10

20

30

40

50

Ir (311) Ir (222)

Untreated

60

2-Theta - Scale

70

80

90

Figure 2. XRD scans of untreated (black), 350°C (red), 450°C (blue) and 550°C (green) heat treated IrO2. Two crystalline phases were identified (Ir metallic and IrO2) which are indicated by their Miller indices. The surface chemistry of untreated and heat treated IrO2 powders was investigated by XPS (Fig. 3). For the pristine samples (i.e., unexposed to the CV accelerated stress test), the Ir 4f regions of the spectra (Fig. 3a), present a small peak shift to lower binding energies with increasing heat treatment temperature; but, in essence, for all samples two characteristic peaks are observed with 3 eV energy difference at ca. 61.7 eV and 64.6 to 64.7 eV. Metallic Ir, which was identified by 11 ACS Paragon Plus Environment


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XRD (Fig. 2), is normally seen at 60.8 eV and generates asymmetric peaks typical for metallic conductors.16,30 The absence of the metallic Ir XPS signal indicates that it is covered by a surface oxide film. The peaks with a doublet separation of 3 eV belong to Ir4+ 4f 7/2 and 5/2, corresponding to anhydrous rutile-type IrO2.30 The shift toward slightly lower binding energies with increasing heat treatment temperature reflects the increased content of crystalline vs. amorphous IrO2. Accelerated CV cycling of the samples produced somewhat wider peaks in the Ir 4f region as a result of Ir3+ presence with peaks at 62.6 eV and 65.6 eV. The oxygen regions of the pristine (uncycled) IrO2 powders (Fig. 3c), changed with increasing heat treatment temperature. Thus, the lattice oxide peak at ca. 530 eV became more pronounced and sharper and a second peak at 531.5 eV developed with increasing temperature due to oxyhydroxide formation. This is similar to previous reports10,16,31 showing the difference in XPS spectra between untreated and heat treated IrO2. Fig. 3b and d present the XPS spectra of the IrO2 samples after being subjected to CV accelerated stress tests. The most significant changes induced by successive potential cycling are observed in the oxygen region. For the untreated oxide, there is a peak broadening due to OH and presence of a new oxygen peak at ca. 532.4 eV (Fig. 3d). These changes are due to hydrated iridium oxyhydroxide formation.31,32 Additionally, with potential cycling, the main oxide peak for IrO2 550 °C shifted toward higher binding energies, from 530 to 532.4 eV (Fig. 3d). The latter peak (at 532.4 eV) is characteristic for O (1s) in surface adsorbed water.30 Previously, Augustynski et al. reported the presence of sulfate ions in electrochemically prepared IrO2 films.33 The XPS survey scans (Fig. S6) did not identify any sulfur present in the IrO2 layers after CV accelerated stress tests.


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64.6 eV

2000

61.7 eV -1

a

4000

IrO2 Untreated 2000

350 450

0

No metallic Ir on surface

550 70

66

64

62

After CV Cycling

1200

IrO2 Untreated

800

60

Binding Energy / eV 529.9 eV

OH

4000

Untreated 350 450 2000 550 534

70

No metallic Ir on surface 68

66

64

532

530

Binding Energy / eV

528

550

536

60

d Lattice Oxide 530.0 eV

After CV Cycling 6400 Untreated

5600

62

Binding Energy / eV

-1

Lattice Oxide

Intensity / counts s

-1

c

64.7 eV

550

Third 7200 O Peak 532.4 eV

6000

536

1600

400

68

61.7 eV

b Intensity / counts s


-1

6000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


OH 531.4 eV 534

532

530

528

Binding Energy / eV

Figure 3. XPS of the Ir region (a and b) and Oxygen region (c and d) of pristine (a and c) and post CV accelerated stress test (b and d) of IrO2: untreated (black), 350°C (red), 450°C (blue) and 550°C (green) heat treated. CV cycling performed between 0.05 to 1.2, 1.4 and 1.6 V vs. RHE, 30 times each at 25 °C in 0.09 M H2SO4. SEM imaging was performed to determine morphological changes from heat treatment of the IrO2 catalysts (Fig. 4). Particles and their aggregates were in general larger after heat treatment at 550 °C (Fig. 4b) than the untreated IrO2 (Fig. 4a). Untreated IrO2 had more small particles dispersed over the Au substrate surface. In contrast, IrO2 heat treated to 550 °C had nearly all of its particles contained within larger aggregates. The incomplete coverage of the Au substrate with 13 ACS Paragon Plus Environment


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IrO2 aggregates is a limitation of investigating drop cast powders. SEM imaging of IrO2 deposits heat treated at 350 and 450 °C (Fig. S4 and S5) showed morphology similar to IrO2 heat treated at 550 °C. Furthermore, SEM images taken after CV accelerated stress tests did not have any apparent differences to those of pristine films (Fig. S4 and S5).

Figure 4. SEM images of pristine (a) untreated IrO2 and (b) IrO2 550 °C drop cast on an Au EQCM crystal. Images were acquired at 138 kx.


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3.1. Cyclic Voltammetry and Relative Capacitance (RC) CVs were obtained concurrently to the frequency response for each IrO2 layer (Fig. 5). The CVs show very little change from the beginning to the end of the stress test. Comparing untreated IrO2 (Fig. 5a) to the heat treated IrO2 (Fig. 5b-d), we can observe heat treatment causes a large decrease in the capacitive current. The area under the CV from 0.5 to 1.0 V vs. RHE was integrated to find the total charge for each catalyst layer (expressed in millicoulombs, mC). The total charge in the above potential region is due to both electric double layer charging and contributions from faradaic charge transfer. Isolating the purely capacitive component is not possible based on the CVs showed in Fig. 5. The total charge was found to be: 13 mC, 4.0 mC, 1.4 mC and 0.8 mC for IrO2 untreated, 350 °C, 450 °C and 550 °C heat treated, respectively. For the untreated IrO2 the total charge was more than 15 times larger than for the 550 °C heat treated sample (Fig. 5). The decrease of the total charge (Fig. 5) with heat treatment of IrO2 is much larger than the BET surface area loss which decreased from 28.2 to 19.0 cm2 g-1(Table 1). The decrease of total charge with increasing heat treatment temperature is caused by a decrease in the surface area of the aggregates, possibly a decrease in the capacitance of the aggregates themselves and a decrease in the coverage of the film on the surface of the gold electrode (as seen in SEM imaging, Fig. 4). Additionally, heat treatment caused a decrease of the peak current at ca. 0.7 V vs. RHE. Previous studies have found that this wave corresponds to the oxidation of Ir3+ to Ir4+.34,35 We propose that thermal oxidation of Ir3+ to Ir4+ during heat treatment decreases the proportion of Ir3+ in the sample. The peaks at ca. 1.4 and 1.1 V vs. RHE were also present on the CVs of an uncoated Au EQCM crystal. This suggests that they are due to Au oxidation and oxide reduction, respectively (Figs. 5 and S9). The size of these peaks could be proportional to the exposed area of Au not covered by IrO2. 15 ACS Paragon Plus Environment


3

1.6 -2

Ir4+

1

b

IrO2 350 °C i / mA cm

i / mA cm

2.4

a

IrO2 Untreated

-2

2

0

0.8

0.0

-1

Ir3+

1.8 0.0

0.3

0.6

-0.8

0.9

1.2

E / V vs. RHE

1.5

1.2 -2

0.6

0.3

Au peaks

0.0

0.6

0.9

1.2

1.5

1.8

1.5

1.8

E / V vs. RHE

1.6

c

IrO2 450 °C

0.0

1.8

i / mA cm

-2

1.2

i / mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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IrO2 550 °C

d

0.8 0.4 0.0

-0.6

-0.4

0.0

0.3

0.6

0.9

1.2

1.5

1.8

0.0

E / V vs. RHE

0.3

0.6

0.9

1.2

E / V vs. RHE

Figure 5. CV response of IrO2 untreated (a) and IrO2 heat treated (350 °C (b), 450 °C (c) and 550 °C (d)) from 1.2 (black), 1.4 (red) and 1.6 V (blue) to 0.05 vs. RHE, 30 times each at 25 °C in 0.09 M H2SO4. The current at 1.6 V vs. RHE decreases with increasing heat treatment temperature. This can be attributed to the loss of OER activity with increasing heat treatment temperature which has been reported previously by several groups.10,17 Additionally, oxidation of amorphous Ir3+ to Ir4+ by heat treatment decreases the formation of an electrophilic O- during OER. Pfeifer et al. found that the presence of electrophilic O- corresponds to increased OER activity. They proposed that this effect is caused by the increased reactivity of O- which promotes O-O bond formation.34 16 ACS Paragon Plus Environment

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3.2. EQCM Response of Heat Treated vs. Untreated IrO2 Interpreting the EQCM response of a porous layer deposited from an ink can be challenging. An observed increase in frequency cannot be automatically attributed to potential dependent mass decrease of the deposited film. It could also be caused by viscoelastic effects, detachment of particles from the film, a chemical change to the film or a decrease in the viscosity of the liquid in the vicinity of the electrode surface.36–39 Fig. 6 shows the frequency response of the untreated and heat treated (at 350, 450 and 550 °C) IrO2 during successive CV tests. Regardless of the IrO2 sample, the first potential sweep (denoted as “fs” in Fig. 6) starting at 1.15 V vs. RHE and going in the cathodic direction, was associated with a decrease of frequency (i.e., mass increase) up to either 0.7 V vs. RHE (for untreated and 350 °C samples) or 1.0 V vs. RHE (for 450 and 550 °C samples). Further scanning to 0.05 V vs. RHE induced a frequency increase (i.e., mass decrease) for all samples starting at approximately 0.7 V vs. RHE. The untreated IrO2 had the smallest overall frequency increase (44 Hz or -310 ng) compared to all the heat treated samples. To determine the cause of this frequency increase, 4 mL electrolyte samples were taken from near the electrode surface during the initial sweep and were tested for Ir content by ICP-MS. No dissolved Ir was detected (limit of detection: 1 μg L-1 or 4 ng dissolved Ir in the sample) in the electrolyte. The total loading of Ir on the EQCM crystal was 28.9 μg. To reach the ICP-MS limit of detection (LOD), 0.00014% of the deposited Ir would need to dissolve into the solution during the first scan from 1.15 V vs. RHE. Additionally, the 4 ng dissolution needed to reach the ICP-MS LOD is 75 times smaller than the smallest first scan mass change of ca. 300 ng in the untreated IrO2 (Fig. 6a). Therefore, the frequency increase is neither due to dissolution of soluble Ir species nor detachment of particles from the surface but can be



Page 18 of 36

associated with potential dependent surface reactions and possibly viscoelastic effects of the particle aggregates. As pointed out by Burke and Scannell, the hydrous Ir oxide surface layer can be regarded as a porous gel or polymer.21 Birss et al. argued that in the case of an electrodeposited IrO2 film viscoelastic effects are negligible.20 However, here we deal with particle aggregates. We speculate that during the first sweep after immersion of the EQCM probe in the electrolyte, the aggregation of particles is not yet stabilized and changes in layer viscosity could take place due to hydration. Such viscosity changes could influence the measured frequency response. Lu et al. investigated the viscoelastic effects of binderless carbon blacks with different aggregate sizes.40 They reported that the interparticle movement (e.g., sliding of the layers relative to each other) contributes to the viscous deformation but overall the elastic character is dominant. Similar effects can be expected for hydrous Ir oxide particle aggregates as well. Kanazawa showed that the resonant frequency increases abruptly with increasing viscosity as the film transitions from elastic to viscous character.41 Outside this transition region, the frequency is constant and independent of viscosity. In the transition region, for a 6 µm thick viscoelastic film with a density of 1200 kg m-3 and shear modulus of 2x107 N m-2 the expected frequency increase is approximately 0.13 MHz.41 The latter value is orders of magnitude higher than the frequency changes measured in the present work even when accounting for possibly thinner film (Fig. 6). Therefore, we propose that the elastic character of the film dominates and the frequency changes are related to intrinsic film mass changes. Regarding surface reactions during the first cycle, the reduction of mainly anhydrous IrO2 at potentials lower than 0.7 V vs. RHE is causing a mass decrease on the electrode according to equation (3):


Page 19 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2𝐼𝑟𝑂2,(𝑠) + 2𝐻 + + 2𝑒 ― →𝐼𝑟2𝑂3,(𝑠) + 𝐻2𝑂

(3)

This could explain the observed first cycle frequency increases below 0.7 V vs. RHE (Fig. 6). As shown by the XRD spectra (Fig. 2), the Ir oxide species have different compositions for the heat treated, crystalline samples as compared to the untreated, amorphous species. The latter has the lowest oxide content (Fig. 2) and, therefore, exhibits the smallest frequency increase (mass decrease) according to equation 3 during the first cycle (Fig. 6).



a

300

-450

60

240

0.3

0.6

0.9

1.2

E / V vs. RHE

c

IrO2 450 °C

1.5

c

0

160

-1200

0

fs

-50 -100 -150

1.8

-400

50

400 800

120

0.3

0.6

0.9

1.2

1.5

1.8

0.3

0.6

0.9

1.2

E / V vs. RHE

1.5

IrO2 550 °C d

1.8

d

-1200 -800

80 -400

40

fs

0 -40

0

400

-80 -120

0.0

500 0.0

-800

100

0

-60

mass / ng f / Hz

150

0.0

fs

0

150 200

-500

60

0

fs

-1000

120

-150

-30

-2000

180

-300

30

0

b

IrO2 350 °C b

-1500

a


f / Hz

-600

mass / ng

IrO2 Untreated

mass / ng

90

f / Hz

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 36

800 0.0

0.3

0.6

0.9

1.2

1.5

1.8

E / V vs. RHE

E / V vs. RHE

Figure 6. EQCM response of untreated (a) and heat treated (350 °C (b), 450 °C (c) and 550 °C (d)) IrO2 during cyclic voltammetry experiments from: 1.2 V (black), 1.4 V (red) and 1.6 V (blue) to 0.05 V vs. RHE. The cycles were repeated 30 times each. “fs” – indicates first potential sweep. Temperature: 25 °C. Electrolyte: 0.09 M H2SO4. Following the first sweep, successive potential cycling coupled with frequency measurements were performed using three start potentials: 1.2, 1.4 and 1.6 V vs. RHE (Fig. 6). The corresponding cyclic voltammograms are shown in Fig. 5. Two observations are made. First, the frequency decreases with each successive cycle for all samples. Second, the potential dependent frequency profiles of the 450 and 550 °C heat treated IrO2 samples are similar from the first cycle through subsequent cycles, whereas in case of the 350 °C heat treated and especially the untreated IrO2,


Page 21 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the frequency profiles changed significantly between the first cycle and the subsequent cycles. The frequency response for the Au EQCM crystal alone was investigated using the same CV cycling protocol to filter out and possible interference from the uncovered Au surface. There was only a small decrease in frequency (ca.-10 Hz or 71 ng) during the first 30 cycles from 1.2 to 0.05 V vs. RHE. Afterwards, the frequency response was stable (i.e., frequency change lower than 5 Hz) during successive cycling from 1.4 or 1.6 V to 0.05 V vs. RHE (Fig. S9). Hence, the observed frequency profile changes during successive cycling (Fig. 6) are intrinsic features of the differently heat treated IrO2 films. The gradual frequency decreases (i.e., mass increase) with each successive cycle is due to chemical process occurring that increase the mass of the adhered film. The XPS response of these films after the CV stress tests (Fig. 3d) shows larger peaks for OH and surface adsorbed H2O suggesting that compositional modifications occurred over time in the film due water uptake and formation of: IrO2xH2O, Ir2O3xH2O and various Ir oxyhydroxide species (e.g., IrO(OH)y). For the untreated sample (Fig. 6a), all successive scans show a frequency increase peak in the potential region 0.6 V to 1.1 V vs. RHE. In contrast, successive scans of the heat treated IrO2 in the same potential region display either a fairly constant frequency (for the sample heat treated at 350 °C, Fig. 6b) or a frequency decrease valley for the 450 and 550 °C treated samples (Fig. 6c and d). In other words, Fig. 6 reveals that increasing the heat treatment temperature of IrO2 above 350 °C increases the apparent mass between ca. 0.6 V to 1.1 V vs. RHE, whereas the untreated IrO2 displays a low apparent mass in the same potential region. This behavior is independent of the start potential and is an intrinsic feature of IrO2 as a function of heat treatment. Furthermore, there is little hysteresis between the anodic and cathodic scanning direction of each cycle (Fig. 7). In between each set of thirty cycles there is a frequency gap before the beginning of the next set 21 ACS Paragon Plus Environment


Page 22 of 36

of thirty cycles. The gap is caused by a five second hold at the starting potential (max potential minus 0.05 V vs. RHE). The final frequency increases at 1.4 and 1.6 V correspond to the Ir4+ to Ir5+/6+ transition and the beginning of the OER (Fig. 6). This causes a further frequency increase from oxygen evolution on the surface of the electrode.32 In Fig. 7, we overlay the final CV scan (1.6 V to 0.05 V vs. RHE) and the corresponding frequency response of Fig. 6. During polarization from 0.05 V to 1.6 V, the frequency of the untreated IrO2 increased by ca. 60 Hz (-425 ng) (Fig. 7b). Birss et al. reported the formation of an iridium oxyhydroxide at low potentials which is oxidized to less hydrated IrO2 and causes a mass decrease when polarizing from 0 to 1.6 V vs. RHE.20 They suggested that the oxyhydroxide Ir3+ species has a higher molar mass than IrO2. The differences in the frequency profiles between untreated and heat treated (especially above 350 °C) IrO2 (Fig. 7d and e) can be explained in relation to the XPS spectra shown by Fig. 3d. As the heat treatment temperature is increased, the increasingly crystalline IrO2 has a higher uptake of surface adsorbed water (as revealed by XPS), a process which in terms of frequency response, competes with the mass changes induced by the redox reaction between Ir3+ and Ir4+ oxyhydroxide species.


Page 23 of 36

a

Au

6

4 0.0

f / Hz

i / mA cm

-2

0.2

2 -0.2 0

i / mA cm

-2

2

10

3.0

0

2.5

-10

2.0

-20

1

-30 0

0.9

-2

b

IrO2 Untreated

0.6

E / V vs. RHE

-40

0.0

0.3

2.0 1.5

0.6

0.9

1.2

E / V vs. RHE

1.5

c

IrO2 450 °C

1.8

c

IrO2 350 °C

-50

1.5

-100

1.0

-150

0.5

-1.0

1.8 50

2.0

0

-200 -250

0.0

0.3

0.6

0.9

1.2

E / V vs. RHE

1.5

d

IrO2 550 °C

0.5

-100

0.0

-150

-0.5

-200

-1.0

-250 1.8

0.3

0.6

0.9

1.2

E / V vs. RHE

1.5

1.0

f / Hz

i / mA cm

f / Hz

-2

0.0

50

-50

-2

-50

-300 1.8

0

1.5 1.0

50 0

-0.5

-60 -2

1.5

0.0

-50

-1

1.2

f / Hz

0.3

f / Hz

3

0.0

i / mA cm

-0.4

i / mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-100 0.5 -150 0.0 -0.5

-200

0.0

0.3

0.6

0.9

1.2

E / V vs. RHE

1.5

-250 1.8

Figure 7. Combined frequency and current responses for (a) background Au EQCM crystal, (b) untreated IrO2, (c) 350 °C heat treated IrO2, (d) 450 °C heat treated IrO2 and (e) 550 °C heat treated IrO2 during the final CV scan from 1.6 V to 0.05 V vs. RHE at 25 °C in 0.09 M H2SO4.



3.3. EQCM Response to Potentiostatic Holds The frequency response of untreated IrO2 was measured under potentiostatic conditions (Fig. 8). The frequency change during 5 minute potential holds at 0.9, 1.2, 1.4 and 1.6 V vs. RHE (but not at 0.05 V vs. RHE) was similar for the pristine and CV stress tested untreated IrO2. When polarized at positive potentials, the frequency response has an initial frequency spike (25 to 75 Hz or -170 to -530 ng) which then decreases over five minutes. This is caused by the oxidation of oxyhydroxide species to lighter IrO2. At 1.6 V vs. RHE, the frequency is lower than at 1.4 V vs. RHE indicating the formation of a heavier layer due to the start of the OER and formation of heavier Ir5+ species. The main difference occurs when polarizing to 0.05 V vs. RHE where the pristine film has a large frequency increase of ca. 900 Hz (-7 μg) while the CV cycled film has a frequency decrease of ca. 200 Hz (1.4 μg). The frequency increase of the pristine film is similar to the first sweep frequency increase shown in Fig. 6 and discussed previously. This effect is not observed in the CV cycled film because of different Ir speciation and water uptake characteristics.

Pristine

0.05 V

-4500 -3000

400

1.2 V 1.4 V

200

0.9 V

0

-1500

1.6 V

0

-200

1500 0

400

800

Time / s

1200

1600

1.4 V

0

1.6 V

-50 -100

b

0

500

1.2 V 0.9 V

1000

-150 -200

Untreated IrO2

-250

After CV Cycling

1500

mass / ng

600

a

Untreated IrO2

-6000


800

f / Hz

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 36

0.05 V 2000

-300 0

400

800

1200

Time / s

1600

2000

Figure 8. Five minute potential holds at 0.9, 1.2, 1.4, 1.6 and 0.05 V vs. RHE with (a) freshly prepared and (b) CV cycled untreated IrO2. Electrolyte: 0.09 M H2SO4 purged with N2. Scan rate: 100 mV s-1. Temperature: 25 °C. 24 ACS Paragon Plus Environment

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3.4. Pt/C with IrO2 Potential Cycling and Constant Potential Polarization The frequency response of carbon and Pt/C containing layers was measured with and without IrO2 (Fig. 9). The frequency increases during the successive CV cycling, which indicates mass decrease of the deposited film. Pt/C alone had the largest mass decrease followed by Pt/C + untreated IrO2, Ketjen black and Pt/C + IrO2 550 °C. Wickman et al. argued that most of the mass loss can be attributed to the dissolution of Pt from a Pt/C catalyst.26 A smaller mass loss contribution from the carbon support was also observed. da Silva et al. investigated the degradation of Pt/IrOx bifunctional catalysts by ICP-MS. Dissolution of both Pt and Ir was observed by ICP-MS during potential cycling (and square wave potential steps) from 0.6 to 1.6 V vs. RHE.42,43 In our case however, Pt dissolution combined with carbon corrosion cannot explain the total observed mass loss of the film because the calculated Pt/C mass decrease (ca. -18 μg) was larger than the total deposited mass (10.3 μg). Additionally, the layers were visually inspected after each experiment and there was no change in the appearance of the film that would indicate that all of the Pt and carbon had been corroded. The Pt/C + untreated IrO2, and Pt/C + IrO2 550 °C, experiments were repeated three times and the calculated decrease in mass was 11.1 ± 1.5 μg and 2.6 ± 1.2 μg respectively. This can be compared to the calculated total solids loading of 10.3 μg. Therefore, in the presence of carbon support viscoelastic effects must be considered as well to explain the observed frequency increase. Lu et al. investigated the viscoelastic properties and their relationship to aggregate sizes of carbon black.40 They found that the viscous character increases and the elastic character decreases with increasing aggregate size. The effect of these changes on the EQCM response has been to some extent investigated using conducting and non-conducting polymers.41,44–46 In the case of the Pt/C film there seems to be an enhanced viscous character induced by extensive potential cycling. The decreasing rigidity of the film could lead to significant



phase shift and possibly dissipation before reaching the outer film surface. In this case, the Sauerbrey equation no longer applies.45

Ketjen

3000

b

2500

-8000

Pt/C

-4000

300

-2000

0

0.0

0.3

c

1600

0.6

0.9

1.2

E / V vs. RHE

1.5


-4400 -2200 0 0.0

0.3

0.6

0.9

1.2

1.5

0

0.0

0.3

0.6

0.9

1.2

E / V vs. RHE

1.5

1.8

3000 1.8

600

-4200

500

-3500

400

-2800

300

-2100

200

-1400

100

-700

-11000

800

0

-3000

d Pt/C+IrO2 (550 °C)

-13200

-6600

400

-6000

500

-500

-8800

1200

-9000 1000

0

0 1.8

Pt/C+IrO2 (unt.)

-12000

1500

0

mass / ng

600


f / Hz

-6000

-18000 -15000

2000

900

-21000

mass / ng

a

1200

f / Hz

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 36

0.0

0.3

E / V vs. RHE

0.6

0.9

1.2

E / V vs. RHE

1.5

0 1.8

Figure 9. Frequency response of (a) Ketjen black, (b) Pt/C alone and with (c) untreated IrO2 and (d) IrO2 heat treated at 550 °C. Catalyst mixtures were 2:1 Pt:Ir metals basis and total solids loading were 2.7, 5.5 and 7.5 μg cm-2 for a, b and both c and d respectively. CV cycled from 1.2 (black), 1.4 (red) and 1.6 V (blue) to 0.05 V vs. RHE. Electrolyte: 0.09 M H2SO4 purged with N2. Scan rate:100 mV s-1. Temperature: 25 °C. Adding IrO2 to Pt/C caused lower frequency increase during successive CV sweeps compared to Pt/C alone. Pt/C + IrO2 550 °C had the smallest frequency increase of all the tested catalysts (Fig. 9). Since the frequency increase cannot be caused solely by Pt dissolution and carbon corrosion, 26 ACS Paragon Plus Environment

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and the addition of IrO2 must also have some impact on the physical properties of the film. The IrO2 particles were well incorporated into the Pt/C particles (Fig. S8) and likely increased the rigidity of the film mitigating the viscous behavior. Fig. 10 shows the frequency responses accompanying the first cycle voltammetry sweeps for Pt/C and Pt/C + IrO2. The complete set of thirty CV cycles with three potential ranges are presented in the Supporting Material (Fig. S7). In case of Ketjen black the frequency increases by ca. 230 Hz during the cathodic sweep and then, during the anodic sweep, after a fairly constant frequency domain up to 1.2 V, decreases by ca. 170 Hz up to 1.6 V (Fig. 10a). The latter frequency drop is due to the formation of oxidized functional groups on carbon coupled with oxide formation on the uncovered Au substrate. The oxidation and reduction peaks at 1.4 and 1.1 v vs. RHE respectively, can be attributed to the exposed Au substrate oxidation and oxide reduction, as discussed previously (Fig. S9 b). Furthermore, for Ketjen black the peak at ca. 0.6 V vs. RHE on the cathodic sweep can be attributed to the two-electron O2 reduction (Fig. 10a). For each of the Pt/C containing layers (Fig. 10b, c and d) the CVs and frequency responses have the same basic shape. During the cathodic sweep there is oxide reduction (both Pt and Au) with the associated frequency increase followed by levelling off in the hydrogen region underpotential deposition region. The anodic sweep begins with a steep frequency decrease suggested previously to be caused by ingress of water into the support.24 It is noted that this frequency decrease was absent in case of the carbon support alone (compare Figs. 10a with 10b, c or d). Due to the presence of Pt the catalyst layer becomes more hydrophilic with enhanced water uptake and strong water adsorption on Pt.



Ketjen

a

0.6

0.6

900

0.0

f / Hz -2 i / mA cm

-2

0.2

1500

Pt/C Pt oxidation / Dissolution

1000

0.4

i / mA cm

b

0.9

0.3

1200 900

0.0

Pt reduction

600

f / Hz

1100

-0.3

800

300

-0.2

-0.6 0.0

1.5

0.3

c

1.0

0.6

0.9

E / V vs. RHE

1.2

1.5

700 1.8

Pt/C+IrO2 (unt.)

0.0

1000

0.6

800 600

-0.5

400

-1.0

f / Hz -2 i / mA cm

-2

0.5 0.0

0.9

0.3

0.6

0.9

1.2

E / V vs. RHE

1.5

0 1.8

d Pt/C+IrO2 (550 °C) 500 450

0.3

400

0.0

350

-0.3

300

-0.6

250

f / Hz

-0.4

i / mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 36

200

-1.5 0.0

0.3

0.6

0.9

1.2

E / V vs. RHE

1.5

0 1.8

-0.9

0.0

0.3

0.6

0.9

E / V vs. RHE

1.2

1.5

200 1.8

Figure 10. The first CV cycle from 1.6 to 0.05 V vs. RHE of (a) Ketjen black, (b) Pt/C alone and with (c) untreated IrO2 and (d) IrO2 heat treated at 550 °C. Catalyst mixtures were 2:1 Pt:Ir metals basis. CVs were recorded in 0.09 M H2SO4 purged with N2 at 100 mV s-1 at 25 °C. Thereafter, the frequency levels off from ca. 0.6 to 0.9 V vs. RHE and increases slightly during Pt oxidation between 0.9 and 1.1 V vs. RHE. This increase is due to the conversion of Pt hydroxides to an oxide monolayer and likely some Pt dissolution. da Silva et al. found that Pt dissolution in Pt/IrOx bi-functional catalysts was at a maximum near 0.6 V vs. RHE.42 Further scanning to 1.6 V produces a frequency decrease as indicated previously due to more extensive surface oxidation of both Pt and the exposed Au substrate.


Page 29 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Constant potential polarization for 300 s each at 1.6, 1.8 and 2.0 V were also applied to Pt/C and Pt/C + IrO2 catalyst layers (Fig. 11). These experiments were carried out to simulate a typical fuel cell voltage reversal event where the cell is starved of fuel and held at a fixed current until a potential limit is reached. The potential holds at 1.6 V vs. RHE were similar for Pt/C alone and Pt/C with untreated IrO2. Starting at 1.8 V vs. RHE, however, Pt/C alone experienced a sharp frequency increase (mass decrease) of 150 Hz or -1300 ng, respectively. The slope of the frequency change increased at 2.0 V vs. RHE (Fig. 11) as a result of carbon corrosion and Pt dissolution. In contrast, the Pt/C + IrO2 displayed a slight frequency decrease (mass increase) at 1.8 V. This difference in the frequency response indicates that instead of carbon corrosion and Pt dissolution, OER is favored in the presence of IrO2 creating a durable catalyst able to withstand voltage spike events. The slight frequency decrease with IrO2 could be due to oxidation of the metallic Ir under the IrO2 surface. The evolved gas bubbles from the Pt/C + untreated IrO2 layer were small, whereas, the evolved gas bubbles from the Pt/C alone were fewer, larger and adhered to the surface. After the test, the electrode was agitated to remove any bubbles that were attached to the surface. No significant change in the end frequency was observed after removal of the bubbles indicating that these frequency increases of the Pt/C alone were not caused by adhered gas bubbles changing the density and viscosity of the electrolyte near the surface. Furthermore, during constant potential hold experiments the viscous behavior of the Pt/C film does not appear to have an effect in contrast to the extensive potential sweep cycles (Fig. 11 vs. 9b). Thus, the persistent mass decrease on Pt/C was caused by carbon corrosion and Pt dissolution at high potentials, whereas the Pt/C + IrO2 was protected by the OER. Fig. S8 shows the EDS mapping of the Pt/C + IrO2 catalyst.



600

Pt/C

-3000 -2000

2.0 V 200

1.8 V 1.6 V

0

-1000

Pt/C + IrO2 Unt

0 300

mass / ng

-4000

400

f / Hz

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 36

600

0

900

Time / s

Figure 11. Five minute potential holds at 1.6, 1.8 and 2.0 V vs. RHE with freshly prepared Pt/C (black) and Pt/C with untreated IrO2 (red). Pt/C with untreated IrO2 had to be stopped after 1.8 V vs. RHE because of damage to the crystal. Electrolyte: 0.09 M H2SO4 purged with N2. 25 °C.

4. Conclusions EQCM was used to investigate the frequency changes of untreated and heat treated IrO2 catalyst powders without or with Pt/C. The combination of Pt/C with IrO2 catalysts is relevant for simulating a PEMFC anode designed for voltage reversal tolerance. The goal is to develop methodologies to better understand the voltage reversal tolerance of fuel cell anode catalysts and more efficiently screen possible catalyst formulations. The untreated and heat treated (at 350, 450 and 550 oC) crystalline IrO2 behaved differently in terms of frequency change during thirty successive potential cycles. This was caused by potential dependent uptake of water and formation 30 ACS Paragon Plus Environment

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of iridium oxyhydroxide layers with different compositions, confirmed by XPS. The exact chemical makeup of the layer was difficult to determine because of the complex nature of the iridium chemistry; but there was no observed sulfur in the XPS scan suggesting that SO42- ions from the electrolyte did not interfere with the frequency responses. The IrO2 layers remain relatively stable under these CV cycling conditions and there is no significant iridium dissolution (confirmed by ICP-MS). Moreover, the frequency responses of Pt/C + IrO2 catalyst layers were investigated during both potential cycling and constant potential polarization and compared to Pt/C or Ketjen black alone. During potential holds at 1.8 and 2.0 V vs. RHE, a large mass decrease was observed in case of Pt/C caused by carbon corrosion and Pt dissolution. The addition of IrO2 prevented the mass loss as shown by the virtually constant frequency, thereby, it protected Pt/C from corrosion (i.e., simulating ex situ the voltage reversal tolerant anode). The major challenge to using EQCM for powder catalyst investigation is producing layers with consistent and repeatable frequency behavior and separating (or minimizing) viscous layer related effects from intrinsic potential dependent mass changes experienced by the catalyst layers. In this work, the IrO2 and Pt/C deposition conditions were tightly controlled and each experiment was repeated to ensure the consistency of the results. It is demonstrated that EQCM could be a rapid and powerful tool to probe the potential dependent frequency changes associated with a myriad of powder catalyst materials. Supporting Information. Contains further characterization of materials including EQCM calibration, XPS survey results, SEM images, STEM images and electrochemical CVs.



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5. Acknowledgements The authors would like to thank Dr. Myles Bos for his XPS interpretation. We acknowledge Xin Zhang and Dennis Hsiao for their STEM and XPS analysis respectively. Financial support from MITACS (Accelerate Fellowship for Ph.D.) and Ballard are respectfully acknowledged. 6. References (1)

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7. TOC Graphic

600

Pt/C

↑ CO2 200

2.0 V

1.8 V 1.6 V

IrO2

300

600

-2000

Oxyhydroxides

CV Cycling

IrO2-1000

500 °C

Pt/C + IrO2 Unt

0 0

↑ O2

-3000

mass / ng

-4000

400 f / Hz

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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↓ Ir3+ ↑ Ir4+

0

900

Time / s


Vibrating Powders: Electrochemical Quartz Crystal Microbalance

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