Supplementary information

Highly active robust oxide solid solution electro-catalysts for oxygen reduction reaction for

Proton Exchange Membrane Fuel Cell and Direct Methanol Fuel Cell Cathodes

Prasad Prakash Patel1, Oleg I. Velikokhatnyi2,3, Shrinath D. Ghadge1, Prashanth H. Jampani2,

Moni Kanchan Datta2,3, Daeho Hong2, James A. Poston4, Ayyakkannu Manivannan4, Prashant N.

Kumta1,2,3,5*

1Department of Chemical and Petroleum Engineering, Swanson School of Engineering,

University of Pittsburgh, Pittsburgh, PA 15261, USA.

2Department of Bioengineering, Swanson School of Engineering, University of Pittsburgh,

Pittsburgh, PA 15261, USA.

3Center for Complex Engineered Multifunctional Materials, University of Pittsburgh, PA 15261,

USA.

4US Department of Energy, National Energy Technology Laboratory, Morgantown, WV 26507.

5Mechanical Engineering and Materials Science, Swanson School of Engineering, University of

Pittsburgh, Pittsburgh, PA 15261, USA.

*Corresponding author: Dr. Prashant N. Kumta (pkumta@pitt.edu)

Department of Bioengineering, 815C Benedum Hall, 3700 O’Hara Street, Pittsburgh, PA 15261.

Tel: +1-412-648-0223, Fax: +1-412-624-3699

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S1. Experimental methodology

Preparation of Electro-catalyst

A two-step preparation approach was employed for the synthesis of solid solution electro-

catalyst, (W1-xIrx)Oy nanoparticles (NPs) (x0.2, 0.3), as reported earlier by the authors[1],

where WO3 NPs were synthesized in the first step, followed by the synthesis of solid solution in

the second step. The procedure is briefly summarized below.

Synthesis of WO3-NPs

Sodium tungstate dihydrate (Na2WO4.2H2O, 99%, Aldrich) was dissolved in D.I. water,

purified by the Milli-Q system (18 MΩ cm deionized water, Milli-Q Academic, Millipore). The

pH of the aqueous solution of Na2WO4.2H2O was then adjusted to 0.5 by adding hydrochloric

acid (HCl, 37%, Aldrich), followed by well-stirring for 1 h. The solution was then heated to

65oC5oC and the temperature was maintained at 65oC5oC for 30 min for ensuring

completion of the reaction. The resultant slurry was then centrifuged, washed repeatedly with

D.I. water and then dried at 50oC for 6 h, yielding the hydrated tungsten trioxide (H2WO4).

H2WO4 was then heat-treated in ultra high-pure (UHP) argon atmosphere (Matheson; 99.99%,

flow rate 100 cm3 min-1) at 623 K for 2 h to synthesize pure WO3 NPs. 623 K was chosen as

temperature for heat-treatment following the results of thermogravimetric analysis of H2WO4

obtained in UHP-Ar atmosphere, as reported earlier by the present authors and shown in Fig. S1.

[1]

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Synthesis of (W1-xIrx)Oy (x0.2, 0.3)

For synthesis of (W1-xIrx)Oy NPs, stoichiometric amount of IrCl4 (Aldrich) was dissolved

in absolute ethanol inside an atmosphere controlled glove box (MBraun Unilab Work station) to

prevent any undesired side reaction with air, followed by the addition of stoichiometric amount

of WO3 NPs synthesized by the procedure mentioned above. The soaked powder was then dried

in a crucible in an oven at 60oC for 2 h to remove ethanol and subsequently heat treated in air at

673 K for 4 h to synthesize (W1-xIrx)Oy (x0.2, 0.3) solid solutions. For comparison of

electrochemical activity of (W1-xIrx)Oy with IrO2, IrO2 NPs were also synthesized by heat-

treatment of commercial IrCl4 (Aldrich) in air at 673 K for 4 h.

Electro-catalyst characterization

Structural characterization

X-ray diffraction

Qualitative phase analysis of (W1-xIrx)Oy electro-catalyst was carried out using X-ray

diffraction (XRD). Philips XPERT PRO system was used for the XRD analysis, employing

CuK ( 0.15406 nm) radiation at an operating voltage and current of 45 kV and 40 mA,

respectively. The Lorentzian and Gaussian contribution from the peak was determined by peak

profile analysis of (W1-xIrx)Oy using the Pseudo-Voigt function. The single line approximation

method was employed for determination of the integral breadth of the Lorentzian contribution.

The contribution from the instrumental broadening and lattice strain contribution was eliminated

from the integral breadth of the Lorentzian contribution and further used to calculate the particle

size of the (W1-xIrx)Oy, using the Scherrer formula.[2, 3]

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Thermal analysis

The thermal characterization of H2WO4 (Fig. S1) was carried out by conducting

thermogravimetric analysis (TGA) using a TGA-DTA machine (Netzsch STA 09PC/4/H/Luxx

TG-DTA). The TGA analysis has been carried out in ultra-high purity Argon (UHP-Ar)

atmosphere from room temperature up to 773 K employing a heating rate of 10oC min-1.

Microstructure analysis

Scanning electron microscopy (SEM) was carried out for study of the microstructure of

(W1-xIrx)Oy electro-catalyst. The quantitative elemental analysis was performed by using energy

dispersive x-ray spectroscopy (EDX) analyzer (attached with the SEM machine). The

homogeneous distribution of elements within particles of electro-catalyst materials without any

phase segregation on a specific site was ensured by performing elemental x-ray mapping. The

quantitative elemental analysis and elemental x-ray mapping was carried out using Philips XL-

30FEG equipped with an EDX detector system with an ultrathin beryllium window and Si(Li)

detector operating at 20 kV. The particle size and the structure of (W1-xIrx)Oy particles was

studied using transmission electron microscopy using JEOL JEM-2100F.

X-ray photoelectron spectroscopy

The oxidation states of W, Ir and O in (W1-xIrx)Oy were studied by performing x-ray

photoelectron spectroscopy (XPS) on the electro-catalyst materials. A Physical Electronics (PHI)

model 32-096 X-ray source control and a 22-040 power supply interfaced to a model 04-548 X-

ray source with an Omni Focus III spherical capacitance analyzer (SCA) was used for XPS

analysis of the catalyst materials. The system was operated in the pressure range of 10 -8 to 10-9

Torr (1.3 × 10-6 to 1.3 × 10-7 Pa). Calibration of the system was carried out by following the

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manufacturer’s procedures, wherein the photoemission lines, Eb of Cu 2p3/2 (932.7 eV), Eb of Au

4f7/2 (84 eV) and Eb of Ag 3d5/2 (368.3 eV) were utilized employing a magnesium anode. The

experimentally determined peak areas were correspondingly divided by the instrumental

sensitivity factors to determine the desired intensities and these intensity values were reported in

this study. The adventitious C 1s peak to 284.8 eV was considered as a reference for the charge

correction.

Electrochemical characterization

A rotating disk electrode (RDE) setup was used for electrochemical characterization of

electro-catalysts for ORR. The electro-catalyst ink consisting of 85 wt.% electro-catalyst and 15

wt.% Nafion 117 (5 wt.% solution in lower aliphatic alcohols, Aldrich) was sonicated and

applied to a glassy carbon (GC) disk (geometric area0.19 cm2). After solvent evaporation, the

GC surface had a thin layer of electro-catalyst, which served as the working electrode. The total

loading of (W1-xIrx)Oy NPs (x0.2, 0.3) was 20 g on 1 cm2 area. The electrochemical

performance of (W1-xIrx)Oy NPs (x0.2, 0.3) for ORR is compared with state of the art

commercial Pt/C (Alfa Aesar) electro-catalyst in this study. Hence, the electrochemical

performance of commercial 40% Pt/C electro-catalyst (Alfa Aesar) was analyzed with Pt loading

of 20 gPt on 1 cm2 area under identical operating conditions. Also, the electrochemical activity

of pure WO3 NPs and IrO2 NPs was also studied under the same identical operating conditions

using total loading of 20 g cm-2. A Pt wire (Alfa Aesar, 0.25 mm thick, 99.95%) was used as the

counter electrode and Hg/Hg2SO4 (XR-200, Hach) that has a potential of +0.65 V with respect to

normal hydrogen electrode was used as the reference electrode. All potential values in this study

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are reported with respect to reversible hydrogen electrode (RHE), calculated from the formula[4-

6]:

ERHE EHg/Hg2SO4+ E0Hg/Hg2SO4+ 0.059pH

ERHE is the potential versus RHE. EHg/Hg2SO4 is the potential measured against the Hg/Hg2SO4

reference electrode. E0Hg/Hg2SO4 is the standard electrode potential of Hg/Hg2SO4 reference

electrode (+0.65 V vs NHE).

Electrochemical characterization was conducted in an electrochemical workstation

(VersaSTAT 3, Princeton Applied Research) using a three electrode cell configuration at 260C

(maintained using a Fisher Scientific 910 Isotemp refrigerator circulator). The cyclic

voltammetry was conducted in N2-saturated 0.5 M H2SO4 electrolyte solution by scanning the

potential between 0 V (vs RHE) and 1.23 V (vs RHE) at scan rate of 5 mV sec-1. The

electrochemical active surface area (ECSA) of (W1-xIrx)Oy NPs (x=0.3) and Pt/C electro-catalysts

is evaluated from the anodic part of the under-potential deposited hydrogen region of CV curves,

obtained in N2 saturated 0.5 M H2SO4 solution, by using the following equation:

ECSAQr

mc

where, ‘Qr’ is the integrated area of the anodic part of under-potential deposited hydrogen region

in the CV curve, ‘m’ is the loading of the iridium/platinum and ‘c’ is the electrical charge

associated with the monolayer adsorption of hydrogen on Ir (220 µC/cm2)/ Pt (210 µC/cm2).[7-9]

0.5 M H2SO4 solution was initially saturated with N2 to remove oxygen present in the solution.

ORR measurement was carried out by performing polarization studies in O2-saturated 0.5 M

H2SO4 electrolyte solution at 260C at rotation speed of 2500 rpm and scan rate of 5 mV sec-1. The

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current density at 0.9 V (vs RHE, the typical potential for assessing electrochemical activity of

electro-catalysts for ORR[10]) in iR corrected polarization curves of electro-catalysts was used

to compare the electrochemical performance of different electro-catalyst materials. The Tafel

plot after iRΩ correction given by the equation a + b log i (plot of overpotential vs log

current, log i) and the corresponding Tafel slope (b) has been used to determine the reaction

kinetics of ORR.

Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) was carried out to determine the charge

transfer resistance (or polarization resistance) (Rct) of electro-catalysts.[2, 3] EIS has been

conducted in the frequency range of 100 mHz-100 kHz (Amplitude 10 mV) at 0.9 V (vs

RHE which is typical potential for assessing electro-catalyst activity for ORR[10]) in O2-

saturated 0.5 M H2SO4 solution at 26oC using the electrochemical work station (VersaSTAT 3,

Princeton Applied Research). The experimentally obtained EIS plot was fitted using the ZView

software from Scribner Associates with a circuit model RΩ(RctQ1Wo), shown in the inset in Fig.

4d where

RΩ = Ohmic resistance, .i.e., resistance of various components including, electrolyte and electro-

catalyst layer[1-3, 5]

Rct = Charge transfer resistance (,.i.e., polarization resistance) [1-3, 5]

Q1 = Constant phase element representing capacitance behavior of the electro-catalyst surface [1-

3, 5]

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Wo is open circuit terminus Warburg element[1-3, 5]

Methanol tolerance test

To study the potential of synthesized electro-catalyst for ORR in DMFC cathodes,

methanol tolerance test was carried out for electro-catalyst by performing polarization in O2-

saturated 0.5 M H2SO4 electrolyte solution with presence of 1 M methanol at rotation speed of

2500 rpm and scan rate of 5 mV sec-1 at 26oC.

Electrochemical stability test

The electrochemical stability of (W1-xIrx)Oy NPs (x0.3) and commercial Pt/C for long

term operation was studied by performing cyclic voltammetry by scanning potential between 0.6

V (vs RHE) and 1.23 V (vs RHE) in N2-saturated 0.5 M H2SO4 electrolyte solution at 26oC at

scan rate of 5 mV sec-1 for 6000 cycles, followed by conducting polarization in O2-saturated 0.5

M H2SO4 solution after 6000 cycles at rotation speed of 2500 rpm and scan rate of 5 mV sec-1.

[10] Elemental analysis of the electrolyte solution was performed after 6000 cycles by

inductively coupled plasma optical emission spectroscopy (ICP-OES, iCAP 6500 duo Thermo

Fisher) to determine the amount of elements leached out into the electrolyte solution from the

electrode providing information about the electrochemical stability of (W1-xIrx)Oy NPs (x0.3).

[1-3, 5]

Membrane electrode assembly preparation and single cell test analysis

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The anode and cathode electro-catalyst ink was prepared consisting of 85 wt.% electro-

catalyst and 15 wt.% Nafion 117 solution (5 wt.% solution in lower aliphatic alcohols, Aldrich).

For the anode, Pt loading of commercial 40% Pt/C (Alfa Aesar) electro-catalyst was 0.2 mgPt cm-

2. For the cathode, the total loading of 0.3 mg cm-2 was used for (W1-xIrx)Oy (x0.3) electro-

catalyst. The electrodes were prepared by spreading the electro-catalyst ink on teflonized carbon

paper. For the single cell testing [1-3, 5], a membrane electrode assembly (MEA) was fabricated

by using a Nafion 115 membrane which was sandwiched between the anode and cathode. The

Nafion 115 membrane was pretreated first with 3 wt.% hydrogen peroxide solution to its boiling

point to oxidize any organic impurities. Subsequently, it was boiled in D.I. water followed by

immersion in boiling 0.5 M sulfuric acid solution to eliminate impurities. Finally, it was washed

multiple times in D.I water to remove any traces of remnant acid. This membrane was then

stored in D.I. water to avoid dehydration. Sandwiching of Nafion 115 membrane between anode

and cathode was carried out by hot-pressing in a 25T hydraulic lamination hot press with dual

temperature controller (MTI Corporation) at a temperature of 125oC and pressure of 40 atm

applied for 30 sec to ensure good contact between the electrodes and the membrane. This MEA

was then used in the single cell test analysis, carried out for 24 h using the fuel cell test set up

obtained from Electrochem Incorporation at 80oC and 0.1 MPa with UHP-H2 (200 ml min-1) and

UHP-O2 (300 ml min-1) as reactant gases.[1-3, 5]

References:

[1] Patel PP, Jampani PH, Datta MK, Velikokhatnyi OI, Hong D, Poston JA, et al. WO3 based

solid solution oxide - promising proton exchange membrane fuel cell anode electro-catalyst.

Journal of Materials Chemistry A. 2015;3:18296-309.

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[2] Patel PP, Datta MK, Velikokhatnyi OI, Jampani P, Hong D, Poston JA, et al. Nanostructured

robust cobalt metal alloy based anode electro-catalysts exhibiting remarkably high performance

and durability for proton exchange membrane fuel cells. Journal of Materials Chemistry A.

2015;3:14015-32.

[3] Patel PP, Datta MK, Jampani PH, Hong D, Poston JA, Manivannan A, et al. High

performance and durable nanostructured TiN supported Pt50–Ru50 anode catalyst for direct

methanol fuel cell (DMFC). Journal of Power Sources. 2015;293:437-46.

[4] Patel PP, Hanumantha PJ, Velikokhatnyi OI, Datta MK, Gattu B, Poston JA, et al. Vertically

aligned nitrogen doped (Sn,Nb)O2 nanotubes – Robust photoanodes for hydrogen generation by

photoelectrochemical water splitting. Materials Science and Engineering: B. 2016;208:1-14.

[5] Patel PP, Datta MK, Velikokhatnyi OI, Kuruba R, Damodaran K, Jampani P, et al. Noble

metal-free bifunctional oxygen evolution and oxygen reduction acidic media electro-catalysts.

Scientific Reports. 2016;6:28367.

[6] Patel PP, Hanumantha PJ, Velikokhatnyi OI, Datta MK, Hong D, Gattu B, et al. Nitrogen and

cobalt co-doped zinc oxide nanowires – Viable photoanodes for hydrogen generation via

photoelectrochemical water splitting. Journal of Power Sources. 2015;299:11-24.

[7] Li B, Higgins DC, Yang D, Lv H, Yu Z, Ma J. Carbon supported Ir nanoparticles modified

and dealloyed with M (M = V, Co, Ni and Ti) as anode catalysts for polymer electrolyte fuel

cells. International Journal of Hydrogen Energy. 2013;38:5813-22.

[8] Li B, Higgins DC, Yang D, Lin R, Yu Z, Ma J. New non-platinum Ir–V–Mo electro-catalyst,

catalytic activity and CO tolerance in hydrogen oxidation reaction. International Journal of

Hydrogen Energy. 2012;37:18843-50.

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[9] Vidakovic-Koch T. Kinetics of methanol electrooxidation on PtRu catalysts in a membrane

electrode assembly: Otto-von-Guericke-Universität Magdeburg, Germany; 2005.

[10] Zhao X, Chen S, Fang Z, Ding J, Sang W, Wang Y, et al. Octahedral Pd@ Pt1. 8Ni Core–

Shell Nanocrystals with Ultrathin PtNi Alloy Shells as Active Catalysts for Oxygen Reduction

Reaction. Journal of the American Chemical Society. 2015;137:2804-7.

Fig. S1: TGA plot of H2WO4 powder in UHP-Ar atmosphere showing the weight loss-

Reproduced from Ref. [1] with permission from the Royal Society of Chemistry.

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Fig. S2: The Tafel plot (for ORR) after iR correction of (W1-xIrx)Oy NPs (x0.2, 0.3) and

commercial Pt/C.

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