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Calculating the Electrochemically Active SurfaceArea of Iridium Oxide in Operating ProtonExchange Membrane ElectrolyzerShuai Zhao

Haoran Yu

Radenka Maric

Nemanja Danilovic

Christopher B. Capuano

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Publication InfoPublished in Journal of Electrochemical Society, Volume 162, Issue 12, Fall 2015, pages F1292-F1298.© Journal of The Electrochemical Society, 2015, Electrochemical SocietyZhao, S., Yu, H., Maric, R., Danilovic, N., Capuano, C., Ayers, K., & Mustain, W. (2015). Calculating the Electrochemically ActiveSurface Area of Iridium Oxide in Operating Proton Exchange Membrane Electrolyzers. Journal Of The Electrochemical Society, 162(12),F1292-F1298. doi: 10.1149/2.0211512jes

Author(s)Shuai Zhao, Haoran Yu, Radenka Maric, Nemanja Danilovic, Christopher B. Capuano, Katherine E. Ayers,and William E. Mustain

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Calculating the Electrochemically Active SurfaceArea of Iridium Oxide in Operating ProtonExchange Membrane ElectrolyzerMustain E William

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F1292 Journal of The Electrochemical Society, 162 (12) F1292-F1298 (2015)0013-4651/2015/162(12)/F1292/7/$33.00 © The Electrochemical Society

Calculating the Electrochemically Active Surface Area of IridiumOxide in Operating Proton Exchange Membrane ElectrolyzersShuai Zhao,a,∗ Haoran Yu,a,∗ Radenka Maric,a,∗∗ Nemanja Danilovic,b,∗∗Christopher B. Capuano,b Katherine E. Ayers,b,∗∗ and William E. Mustaina,∗∗,z

aDepartment of Chemical and Biomolecular Engineering, University of Connecticut, Storrs, Connecticut 06269, USAbProton OnSite, Wallingford, Connecticut 06492, USA

Iridium oxide is one of the most common anode catalysts in commercial proton exchange membrane (PEM) electrolyzers becauseof its strong mix of high activity and stability under oxygen evolution reaction (OER) conditions. Unfortunately, benchmarkingiridium oxide OER catalysts has proven difficult since IrO2 cannot undergo proton underpotential deposition like platinum andother transition metal eletrocatalysts, making it difficult to estimate the electrochemically active surface area (ECSA), as well asOER specific and mass activity. In this work, we propose a method to calculate the ECSA of iridium oxide in an operating PEMelectrolyzer. A universal constant, 596 (± 21) μC/cm2, was obtained from the correlation of pseudocapacitive charge and ECSA ofiridium oxide. In the membrane electrode assembly (MEA), the calculated ECSA (1.81 (±0.065) m2 over a 25-cm2 geometric area)showed an iridium oxide catalyst utilization of ∼93%. Additionally, the IrO2 OER specific and mass activities at 80◦C, 1.6 V in anoperating PEM electrolyzer were 0.401 (±0.014) mA/cm2 and 132 mA/mg, respectively.© 2015 The Electrochemical Society. [DOI: 10.1149/2.0211512jes] All rights reserved.

Manuscript submitted July 13, 2015; revised manuscript received August 12, 2015. Published August 27, 2015. This article is aversion of Paper 1454 from the Phoenix, Arizona, Meeting of the Society, October 11–15, 2015.

One of the most common methods to calculate the electrochem-ically active surface area (ECSA) of a catalyst is to collect a cyclicvoltammogram (CV) in N2-saturated electrolyte and measure the hy-drogen adsorption charge (Qr) in the negative-going scan after correc-tion for double-layer charging.1 This works for platinum because Pthas the ability to undergo hydrogen underpotential deposition (Hupd).The principle behind the Hupd method is that the charge under thevoltammetric peaks for hydrogen adsorption is rooted in the attach-ment of one hydrogen atom on each metal atom on the surface. Theequation for calculating the ECSA of Pt is shown in Equation 1:1

EC S A = Qr

m × C[1]

where m is the Pt mass loading, and C is the charge of full monolayercoverage of H atoms onto clean polycrystalline Pt (210 μC cm−2).This method has been validated versus CO adsorption and Cu under-potential deposition methods for measuring the Pt ECSA2 and hasbeen extended to other metals including Rh, Ir and Ni.

However, iridium oxide does not have access to the same mecha-nism for underpotential deposition of H onto its surface, which makesit difficult to determine the ECSA. This has significant practical im-plications because iridium oxide is a very common anode catalystmaterial for the oxygen evolution reaction (OER) in the proton ex-change membrane (PEM) electrolyzer. The inability to measure theECSA makes it very difficult to benchmark in situ anode catalyst per-formance including catalyst utilization and specific activity. In 1990,Savinell et al.3 found area-pseudocapacitive charge correlations forruthenium dioxide and iridium dioxide electrodes on titanium sub-strates using a zinc ion adsorption method. Though their methodwas able to correlate mass loading with ECSA and pseudocapacitivecharge, they were not able to explain why the kinetic rate constant forFe2+/Fe3+ redox couple on IrO2 was one order of magnitude belowthe expected value using the estimated ECSA, suggesting that theresult was systems-dependent, not a universal constant. Additionally,their electrolyte solution was HCl, which does not well mimic theperfluorosulfonic acid environment of commercial MEAs. Both limi-tations called into question whether this method could be extended toelectrocatalyst operation in state-of-the-art commercial MEAs.3

In this work, we improve upon the Savinell method by introduc-ing a more accurate zinc concentration detection technique, UV-Visspectrophotometry, and performing the experiments in sulfuric acidelectrolytes. We also apply the method to two distinct electrocatalyst

∗Electrochemical Society Student Member.∗∗Electrochemical Society Active Member.

zE-mail: mustain@engr.uconn.edu

systems (electrochemically deposited IrOx thin film and IrOx powderfilm), showing that the results are not microstructural or systems-dependent. We also extend the method to an operating PEM elec-trolyzer membrane electrode assembly (MEA) from Proton OnSite.For the first time we are able to calculate a definitive ECSA constantfor IrO2, and determine the surface utilization of IrO2 as well as theOER specific activity.

Experimental

Before applying to IrO2 in an operating electrolyzer MEA, themethod was developed and validated on both well-defined electrode-posited surfaces and powder catalysts ex situ. The overall experimentalflowsheet to determine the ECSA of IrO2 oxygen evolution reactioncatalysts is shown in Diagram 1.

Both electrodeposited IrOx and IrOx powder catalysts were pre-pared and physically characterized. IrOx mass loadings were deter-mined. The electrodes were then introduced to a three-electrode elec-trochemical cell to collect CVs in order to calculate pseudocapacitivecharge. The resulting electrodes were soaked into solutions containingzinc cations to allow zinc adsorption onto the IrOx surfaces. The trueex-situ ECSA was calculated by multiplying the surface area occu-pied by each zinc cation by the total number of adsorbed zinc cations,which was determined by UV-Vis spectrophotometry. Details of eachstep are given in the following sections.

Electrodeposition of Ir and IrO2 thin films onto polycrystallinegold electrodes.— A 5 mm-diameter disk-type gold working elec-trode (Pine Research Instrumentation) was polished to a mirror finishusing a 0.05 μm alumina-particle suspension (Buehler) on a moist-ened polishing microcloth (Buehler) and washed ultrasonically withultra-pure 18.2 M� deionized (DI) water (Millipore Direct-Q 3UVpurification system) for 4 min prior to each experiment. A Pt flagand Hg/HgSO4 electrode (CH Instruments) were used as counter andreference electrodes, respectively. The electrode was first cleaned bycycling the potential between 0 V and 1.5 V at 200 mV/s for 50 cyclesin 0.5 M H2SO4 (Acros Organics) to remove any surface impurities.

The electrode was transferred to a de-aerated 0.5 M H2SO4/0.04mM H2IrCl6 solution (Dihydrogen hexachloroiridate (IV) hydrate(99.9%), Fisher Scientific) for Ir electrodeposition. Ir deposition wascarried out in a custom-built three-electrode electrochemical cell(Adams & Chittenden Scientific Glass) in N2 atmosphere (by bubblingwith UHP grade nitrogen (Airgas) for 20 minutes before operation) toavoid immediate Ir oxidation. Ir was electrodeposited by chronoam-perometry (Autolab PGSTAT302N Potentiostat, Eco Chemie) withoutrotation at 0.2 V vs. the normal hydrogen electrode (NHE) for various

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Cover IrO surface

with Zn

Introduce EBT to

after adsorption

Cover IrO surface

with Zn

Introduce EBT to

after adsorption

IrO2 on Au working electrodes

Pseudocapacitive charge calculation from CVs

x

2+

Zn2+

solution preparation

zinc solution Concentration Change determined by UV -Vis

Mass loading calculationfrom film preparation

Relationship between zinc surface area and charge and

mass loading

Diagram 1. Experimental procedure flowsheet to link ex-situ pseudocapacitive charge to the true iridium oxide ECSA.

lengths of time in order to achieve different loadings. IntermediateCVs during electrodeposition were collected to monitor the growthof the Ir particles by observing the increase of the hydrogen evolu-tion current with increased cycle number. All potentials are reportedversus NHE.

After Ir deposition, the electrode was washed with a copiousamount of deionized water to remove surface IrCl6

3− and IrCl62−,

and transferred to a 0.5 M H2SO4 electrolyte saturated with nitrogen.CVs were carried out between 0.05 and 1.5 V at 200 mV/s for 50cycles to oxidize the surface Ir to IrOx. After oxidation, the electrodewas rinsed with copious DI water and dried in air.

CVs for calculating the IrOx pseudocapacitive charge were col-lected between 0.0 and 1.5 V in N2-saturated 0.5 M H2SO4 electrolyteat a scan rate of 50 mV/s. Then, OER polarization curves for IrOx

were collected by scanning the potential from 1.0 V to 2.0 V at arate of 5 mV/s while rotating the disk electrode (RDE) at 1600 rpm.The current density at 1.6 V from the oxygen evolution polarizationcurve was used to determine the specific and mass activities of thecatalyst.

Physical characterizations of electrodeposited Ir and IrO2 thinfilm.— The surface morphology of the metal and metal oxide was in-vestigated using a FEI Quanta 250 FEG scanning electron microscope(SEM) with a field emission source and images were captured usingan Everhart-Thornley SE (secondary electron) detector with an elec-tron accelerating voltage of 20 kV, a 3.0 nm spot-size and a sampleheight of 10 mm. The surface elemental distribution was explored byEnergy Dispersive X-ray Spectroscopy (EDS) with a Genesis ApexSystem from EDAX, AMETEK, Inc., using an electron acceleratingvoltage of 20 kV and a working distance of 10 mm. X-ray photo-electron spectroscopy (XPS) was performed using a Physical Elec-tronics multiprobe with a Perkin–Elmer dual anode X-ray source anda Kratos AXIS-165 surface analysis system to examine the surfacecomposition.

Adsorption of zinc cations onto IrOx catalysts.— The adsorptionmechanism behind this method is essentially an ion-exchange adsorp-tion on hydrated surfaces: water reacts with the iridium oxide surface,adding hydrogen to the oxygen atoms, and hydroxyls to the metals,resulting in a surface of metal-hydroxides, and bridging oxygens.4

Surface complexation of the surface OH groups takes place throughthe release of acidity. At pH values above the point of zero charge,the surface has a net negative or anionic charge, attracting cationsand promoting cation exchange reactions, including the absorption ofZn2+, which is shown in Equation 2. After adsorbing zinc ions ontoiridium oxide, the zinc surface area can be correlated with ex-situpseudocapacitive charge on both electrodeposited IrOx and powder

IrOx catalysts, obtaining a constant similar to the conversion factorfor Pt catalysts to estimate the ECSA of iridium oxide through a simpleCV.

OH

OH

+ Zn Zn2+ Ir

O

O+ 2H

+Ir [2]

An ammonium chloride solution containing zinc cations was ob-tained by dissolving white zinc oxide powders (Fisher Scientific)into 0.35 M hydrochloric acid (37%, Fisher Scientific). Ammoniumhydroxide (28%, Fisher Scientific) was added as the source of theammonium ions until the pH was between 9 and 10 in order to coverthe surface with anionic charge to attract cations. The resulting con-centration of zinc cations was 0.045 mM. The oxidized electrodewas immersed into this solution overnight to allow the zinc cationadsorption on IrO2 to reach equilibrium.

Quantifying zinc adsorption by UV-Vis.— Because the adsorptionof zinc ions onto the electrode results in very small changes in its con-centration, titration3 may not be the most reliable method to quantifythe zinc ions. Therefore, the concentration change was determinedby UV-Vis spectrophotometry. UV-Vis measures the ultraviolet orvisible light absorbed by molecules containing π-electrons or non-bonding electrons to determine the molecule concentration in thesolution.

A standard absorption intensity-zinc cation concentration curvewas determined by measuring the adsorption intensities of solutionswith known zinc cation concentration (pH and concentrations of otherions were kept the same). Before each test, the solutions were joinedby complexing indicator Eriochrome Black-T (EBT, [1-(1-hydroxy-2-naphthol azo) 6-nitro-2-naphthol4-sulfonic acid sodium salt]). TheEBT concentration was held constant at 0.005 mM greater than thehighest concentration of zinc cations tested. The indicator is blue inbasic solutions itself but turns pink-red when it chelates with Zn2+

ions, turning the solutions purple. Therefore, the purple intensity canbe correlated with the number of zinc cations in the solution.

After obtaining the standard UV-Vis intensity-zinc cation concen-tration curve, it was straightforward to find the number of zinc cationsadsorbed onto the IrO2 surface by dipping the electrode into a zinccation solution and then detecting the intensity difference of the pur-ple solution before and after adsorption. The ECSA of each electrodewas determined by multiplying the total number of adsorbed zinc ionsby 17 Å2 per zinc ion.3,5

Preparation of IrO2 thin films from powder catalysts onto poly-crystalline gold electrodes.— To extend the Zn-adsorption ECSA

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Figure 1. (a) Chronoamperometry of Ir deposition; (b) Cyclic voltammetry of Ir deposition; (c) Cyclic voltammetry of clean Au electrode; (d) Ir surface oxidationby cyclic voltammetry.

method to commercially relevant catalysts, IrOx thin-film electrodeswere made from IrOx catalysts obtained from Proton OnSite. Beforethe film preparation, the IrOx powders were physically character-ized. SEM was used to investigate the surface morphology. Nitrogenadsorption analysis was performed with a Micromeritics 2020 at 77K and a dose of 3 cm3 (degas at 105◦C for 12 h) to find the BETsurface area of the powders. The pore size distribution of the catalystpowders were obtained from the Barret, Joyner and Halenda (BJH)method built into the ASAP 2020 V 3.04 operating software.

Preparing thin film electrodes from the IrOx powders began withpreparing a 24% isopropanol solution by mixing 6 mL of isopropanoland 19 mL of DI water in a 25 mL volumetric flask. Next, 18.5mg of IrOx powder was placed into a 50 mL volumetric flask andthe 25 mL of 24% isopropanol solution was added. Finally, 100μL of 5 wt% Nafion ionomer DE-520 (DuPont) was added to theIrOx/isopropanol solution, resulting in an ionomer:catalyst ratio (wt)of 5:1. The catalyst ‘ink’ was then placed in an ice bath and sonicatedfor 30 minutes. Droplets (20–40 μL for different mass loading) ofthe dispersed ink were pipetted onto the mirror-finish gold electrodeso that it completely covered the gold electrode. The ink droplet wasdried in air at room temperature under rotation at 300 rpm for 30 min-utes, which yielded a black thin film that homogeneously covered theentire surface of the electrode. The catalyst loading on the electrodewas 0.075–0.15 mg/cm2. In order to fully oxidize the catalyst surface,CVs were carried out between 0.05 and 1.5 V at a scan rate of 200mV/s for 50 cycles, identical to the electrodeposited IrOx in Electrode-position of Ir and IrO2 thin films onto polycrystalline gold electrodessection.

MEA preparation and IrO2 ECSA calculation.— The IrOx anodeCVs were conducted using Fuel Cell Technologies hardware modi-fied for electrolysis. The graphite flow field and the gas diffusion layer(GDL) on the oxygen evolution side of the cell were replaced with atitanium flow field and titanium GDL for stability at high potentials.Catalyst coated membranes with 25 cm2 active area were providedby Proton OnSite. The cathode material was Pt black and the anodematerial was IrOx. The weight ratio of ionomer (5 wt% Nafion disper-

sion) to catalyst was 1:1. The single-cell tests were carried out at roomtemperature with fuel supply control through a Teledyne Medusa RDfuel cell station (Teledyne Energy Systems, Inc.). CVs were obtainedwith a Solartron SI 1287 Potentiostat. The electrolyzer was operatedby circulating excess N2 saturated ultrapure water through the anodeside of the cell with a small pump, and feeding the cathode side with100% humidified hydrogen at 0.5 L/min.

Results and Discussion

Electrodeposition of Ir and IrO2 thin films onto polycrystallinegold electrodes.— Sawy et al. studied the effect of potential on Ir elec-trodeposition on gold substrates in H2IrCl6 solution.6 They observedthe maximum deposition rate (measured by in situ quartz crystal mi-crobalance) was achieved from 0.1 V to 0.2 V because the rate-limitingstep leverages Hupd as the reducing agent for IrCl6

3− to metallic Ir.They also found that the current efficiency of Ir electrodeposition isclose to 100% at 0.2 V when the H2IrCl6 concentration in solution is≤0.05 mM. The insight from these findings guided the Ir depositionbath in this work; the deposition potential was set at 0.2 V, and theH2IrCl6 solution concentration was 0.04 mM.

The chronoamperometric response of the working electrode duringIr deposition is shown in Fig. 1a. The successful deposition of Ir ontothe gold electrode is evident by the increasing hydrogen evolution andoxidation current from CVs (Fig. 1b) collected intermediately duringthe deposition, because the gold is a very poor hydrogen evolutioncatalyst in acid media as shown in the CV in Fig. 1c. After cleaning thedeposited Ir surface with copious DI water, the surface Ir was oxidizedto iridium oxide in 0.5 M H2SO4 electrolyte by sweeping the potentialup to 1.5 V. The oxidation of Ir was accompanied by appearance oftwo characteristic redox peaks at 0.9 V and 1.4 V as well as thedecreasing current density of the hydrogen adsorption/desorption atlow potentials with increased cycle number, which are shown in theCVs in Fig. 1d.

Physical characterization of electrodeposited Ir and IrOx thinfilms.— The as-electrodeposited metallic Ir surface was silver in color

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Figure 2. (a) SEM image of electrodeposited Ir surface; (b) (c) (d) SEMimages of electrodeposited IrOx surface; (e) EDS analysis of image 2d.

Figure 3. XPS spectra of electrodeposited IrOx: Ir 4f spectrum.

and had a near mirror finish. The morphology of this surface was ob-served under SEM, Fig. 2a. The surface had a high density of irregularsurface defects around a few hundred nanometers and many smaller-sized pores with a diameter of approximately 50 nm. The yellowishoxidized IrOx surface in Figs. 2b and 2c showed that there were poresapproximately 100∼300 nm in diameter caused by surface crackingand defects, and other deep pores around 50 nm in diameter. This semi-porous structure led to linearly increased surface area with increasedIrOx loading.

The approximate elemental composition was found by EDS.Fig. 2d showed the surface of IrOx with adsorbed zinc ions. FromFig. 2e (EDS map for Fig. 2d), it can be seen that iridium and oxygenwere homogeneously distributed. Significant zinc could also be seenin the area, even after a thorough cleaning with DI water, showing thatZn was very strongly adsorbed on the surface.

The composition of the IrOx surface was investigated by XPS. Thehigh-resolution spectra for Ir 4f in Fig. 3 were calibrated to the graphite

C 1s peak at 284.6 eV. The Ir 4f spectra were deconvoluted into twodoublets (four peaks). The binding energies of the first deconvoluteddoublet peaks (green and purple lines, respectively) correspondedto that of the Ir4+ 4f5/2 and Ir4+ 4f7/2. The second doublet at lowerbinding energy (orange and blue lines, respectively) was consistentwith metallic Ir 4f5/2 and Ir 4f7/2. Approximately 28 atom % of the nearsurface Ir in the thin film existed in the Ir4+ state with the balance beingIr. The penetration depth of the XPS X-ray probe is 8–10 nm, which isapproximately 23 atomic layers. The electrolyte is only exposed to theoutmost layer. Therefore, the oxidation of the bulk relies heavily onoxygen solid-state diffusion and it is most likely that the near surfaceIr was nearly completely oxidized to IrO2, but the bulk was not.

Relationship between the electrochemically active surface areaand pseudocapacitive charge for electrodeposited IrOx thin films.—The standard UV-Vis intensity-Zn2+ concentration curve is plottedin Fig. 4a. The Zn2+ concentration range for the standard curve was10−6–10−5 M. As expected, the amount of absorbed light proportion-ally decreased with an increase in the zinc concentration. The standardcurve provided a reliable way to estimate the quantity of zinc ions ad-sorbed onto the metal oxide surface by detecting the intensities of thezinc/EBT solution before and after the electrode was immersed.

The pseudocapacitive charge of each catalyst was determined byintegrating the CV from 0.3 V to 1.25 V and dividing by the scan rateof 50 mV/s.7–9 The mass loading was determined by integrating the de-position curve (Fig. 1a). The pseudocapacitive charge has been shownto be a useful relative measure of the active surface area of iridiumoxide.10 The correlation of zinc surface area with pseudocapacitivecharge and zinc surface area with oxide loading are shown in Figs.4b and 4c, respectively. The linearity of the ECSA vs. loading plotshowed that the bulk and surface structure of the electrodeposited elec-trode remained similar. The correlation of ECSA vs. charge showedthe potential promise to find a constant for the IrO2 system similarto the conversion factor11–13 in calculating the ECSA of Pt electro-catalysts. The ECSA-charge and ECSA-loading correlations for theelectrodeposited IrOx films were 1776 (±46) cm2/C and 34.2 (±1.3)m2/g, respectively.

Additionally, the current density at 1.6 V from the OER polariza-tion curve (Fig. 4d) was used to determine the specific and massactivities of the electrodeposited IrOx catalyst, which were 0.477(± 0.012) mA/cm2 and 163 mA/mg, respectively.

Physical characterization of IrOx powder catalysts.— As-receivedIrOx powders were observed under SEM and are shown in Figs.5a and 5b. A single IrOx particle size around 100 nm in diameterwas observed. Most of the particles existed as agglomerates withthe diameter around a few hundred nanometers. The pores betweenagglomerates ranged from tens to several hundreds of nanometers.In order to obtain an accurate pore size distribution and BET surfacearea, nitrogen adsorption was performed. The pore size distributionof these particles is shown in Fig. 5c. The majority of the pores weremesopores from 2∼50 nm in diameter, which are most favorablein electrochemical systems, not only because they are large enoughto be accessible to electrolyte and reactants but small enough tosignificantly increase the surface area of catalysts.14 Less than onethird were micropores, which are not typically electrochemicallyaccessible.15 The cumulative surface area plot, Fig. 5d, showed thatthe mesoporous and macroporous surface area was about 46.6 m2/g,71% of the total BET surface area (65.6 m2/g).

Extending the IrOx ECSA method to in situ powder catalysts.—The as-received IrOx powders were mixed into an ink to be dispersedonto Au disk electrodes. The catalyst layers were oxidized by CVbetween 0.05 and 1.5 V (Fig. 6a) in 0.5 M H2SO4. Then, the electrodeswere treated identically to the electrodeposited IrOx electrodes toobtain the ECSA vs. charge and ECSA vs. loading correlations.

Figs. 6b and 6c show the correlations of the zinc surface areawith pseudocapacitive charge and mass loading of the powders,1663 (±97) cm2/C and 34.8 (±1.3) m2/g, respectively. These are

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Figure 4. (a) Standard intensity-concentration curve; (b) Relationship between the zinc surface area and pseudocapacitive charge; (c) Relationship between thezinc surface area and catalyst mass loading; (d) Polarization curve of IrOx for electrodepostion system at 5 mV/s, 1600 rpm.

comparable to the values of the electrodeposited samples. The ECSAwas 34.84 m2/g, 53% of the total BET surface area (65.6 m2/g) and75% of the mesoporous and macroporous (expected electrochemi-cally accessible) area, which may be caused by poor infiltration of theelectrolyte into relatively deep pores. The specific and mass activities

of the in situ powder catalyst were also similar to the electrodepositedsamples, though slightly lower, 0.418 (±0.024) mA/cm2 and 145mA/mg, respectively.

It should be noted that both the ECSA-charge and ECSA-loading slopes for the powdered film were close to those of the

Figure 5. (a) (b) SEM images of IrOx powder catalysts; (c) Pore size distribution of IrOx powder catalysts; (d) Cumulative surface area of IrOx powder catalysts.

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Figure 6. (a) CV oxidation of IrOx powders; (b) Relationship between the zinc surface area and pseudocapacitive charge for catalyst powder electrode;(c) Relationship between the zinc surface area and catalyst mass loading for catalyst powder electrode; (d) Relationship between the zinc surface area andpseudocapacitive charge for two systems; (e) Cyclic voltammetry of IrOx anode material in MEA; (f) OER polarization curve of IrOx anode material in MEA.

electrodeposited IrOx film, despite the facts that considerable differ-ences existed in the loading and material morphology. This suggeststhat a universal ECSA-charge constant may exsit for polycrystallineIrOx in sulfonic acid systems. Therefore, the data from the powder andelectrodeposited IrOx was combined (Fig. 6d), yielding a “universal”ECSA-charge constant for IrOx of 1681 (±60) cm2/C (or 596 (±21)μC/cm2, to be consistant with Ptupd convention).

The experimental correlation above was validated by comparingthe electrokinetic parameters of the ferrocyanide/ferricyanide (FEFI)redox couple for both a polycrystalline platinum disk and the IrOx

powder electrodes. The FEFI redox couple was selected because itis a simple outer shell electron transfer, which means facile kineticswhere the base metal should have minimal influence on the observedactivity. This means that one would expect the kinentic rate constantand the transfer coefficient for the FEFI to be very close to one anotherfor the Pt and IrOx materials if true ECSA was determined.

The linear sweep voltammogram for the FEFI was collected at1600 rpm at 5 mV/s in N2 saturated 0.01 M K3Fe(CN)6, 0.01 MK4Fe(CN)6, 0.5 M H2SO4 solution between 0 and 1 V. The rateconstants were calculated based on the true surface area (Hupd region

for Pt electrode, and the proposed method for the IrOx electrode). Therate constant of the FEFI redox couple for the Pt electrode, 4.62∗10−3

cm/s, was close to literature value, 4.7 (±0.07)∗10−3 cm/s.16 Therate constant based on the calculated IrOx ECSA, 4.20∗10−3 cm/s,was very close to the Pt disk value, though differences existed on theelectrode surfaces. Also, the transfer coefficients for the reduction ofFe(CN)6

3+ for the Pt disk and IrOx electrodes were nearly identical,0.312 and 0.338, respectively, and reasonably close to the literaturevalue, 0.23 ± 0.03.16

The expected similarity of the Pt and IrO2 FEFI rate constantsfurther demonstrated the improvement of our method over Savinell’sstudy. Since the measurment must quantify an essetially infinitesimalconcentration change (�μM), the measurement sensivity is very im-portant. For UV-Vis, the sensitivity is mostly machine-dependent andthe method can easily detect the color change related to concentrationin this work. However, the error in titration experiments is mostlyhuman-based and even small variations in determining the end pointfor the color change from blue to pink can yield large differences inthe measured Zn adsorption. In addition, Savinell’s work used HClto measure the FEFI rate constants and transfer coefficients though

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F1298 Journal of The Electrochemical Society, 162 (12) F1292-F1298 (2015)

the psuedocapacitive charge was determined in H2SO4. The differ-ing electrolytes are important because Pt is well known to stronglyspecifically adsorb Cl− and IrO2 can be used as the HCl oxidationcatalysts17 and also adsorbs Cl−18. Therefore, it is not unexpect thatthese two effects operating in concert would result in the one order ofmagnitude difference between the theoretical and experimental valuein Savinell’s method.

Extending the IrO2 ECSA method to electrolyzer MEAs.— Next,the IrOx ECSA was determined in situ, in an operating PEM elec-trolyzer, and the specific and mass activity of the catalyst were de-termined. CVs were collected at a 50 mV/s scan rate with 100%humidified H2 fed to the Pt electrode and nitrogen saturated liquid DIwater fed to the IrOx electrode, Fig. 6e. The voltammogram achieveda steady state after around 50 cycles. The shape and characteristicpeaks for IrOx in the electrolyzer were slightly different from the onescollected in the three-electrode RDE setup. Most notably for the elec-trodeposited IrOx CV shown in Fig. 1d, characteristic redox peaksat around 0.9 V, corresponding to the Ir(III)/Ir(IV) solid state redoxtransitions,19 were profound. In the IrOx powder catlayst (Fig. 6a), twosimilarly broad peaks appeared at 0.9 and 1.4 V; however, the prepeakat around 0.8 V preceding the Ir(III)/Ir(IV) redox transition (as shownin Fig. 1a) disappeared. In the MEA, the two peaks at 0.9 and 1.4 V ob-served with the previous two systems were merged together in the CV.Additionally, the potential window was shifted by 0.3 V. In the ex-situtests, the integrated potential window was 0.3–1.25 V. In the MEA,the Pt electrode acts as a quasi reference electrode, shifting the in-tegratable voltage limits to 0.6–1.55 V. The integration limits wereselected based on the position of the IrOx characteristic peaks.

The ECSA obtained from the pseudocapacitive charge was 1.81(±0.065) m2, and specific ECSA was 32.55 m2/g. Comparing withthe ex-situ specific ECSA in the operating electrolyzer, the surfaceutilization was over 93%. Finally, the specific and mass activities ofthe catalyst in the electrolyzer at 80◦C were determined by the currentdensity at 1.6 V in the polarization curve (Fig. 6f), 0.401 (±0.014)mA/cm2 and 131.82 mA/mg, respectively, only slightly less than theex-situ activity.

Conclusions

A method was demonstrated to calculate the ECSA of iridium ox-ide as an anode catalyst material for the oxygen evolution reactionin an operating PEM electrolyzer, which showed promising results atthe bench scale. The ECSA was estimated using the total area of theadsorbed zinc ions onto the metal oxide ex-situ. The ECSA demon-

strated good linearity with the pseudocapacitive charge for both elec-trodeposited and powder IrOx films. The experimental ECSA-chargeconstant for IrOx was 1681 (±60) cm2/C (or 596 (±21) μC/cm2).The final application of this method to the in situ ECSA calculationof IrOx in PEM electrolyzer gave the estimated ECSA, 1.81 (±0.065)m2 over a 25-cm2 geometric area. The catalyst utilization in the MEAwas >93%, and the specific and mass activities were 0.401 (±0.014)mA/cm2 and 132 mA/mg at 1.6 V, respectively.

Acknowledgments

This work was funded by Department of Energy SBIR grant DE-SC0009213. The authors also acknowledge Proton OnSite for materialsupply, and the Institute of Material Science and the Center for CleanEnergy Engineering at the University of Connecticut for free use ofthe physical characterization equipment.

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