Articleshttps://doi.org/10.1038/s41929-020-0475-4

Selective electrocatalysis imparted by metal–insulator transition for durability enhancement of automotive fuel cellsSang-Mun Jung   1,9, Su-Won Yun2,9, Jun-Hyuk Kim2,9, Sang-Hoon You1, Jinheon Park1, Seonggyu Lee3, Seo Hyoung Chang4, Seung Chul Chae5, Sang Hoon Joo   6, Yousung Jung   3, Jinwoo Lee   3, Junwoo Son   1, Joshua Snyder7, Vojislav Stamenkovic8, Nenad M. Markovic8 and Yong-Tae Kim   1 ✉

1Department of Materials Science and Engineering, Pohang University of Science and Technology, Gyeongbuk, Republic of Korea. 2Department of Energy System, Pusan National University, Busan, Republic of Korea. 3Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, Daejeon, Republic of Korea. 4Department of Physics, Chung-Ang University, Seoul, Republic of Korea. 5Department of Physics Education, Seoul National University, Seoul, Republic of Korea. 6Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan, Republic of Korea. 7Department of Chemical and Biological Engineering, Drexel University, Drexel, PA, USA. 8Materials Science Division, Argonne National Laboratory, Lemont, IL, USA. 9These author contributed equally: Sang-Mun Jung, Su-Won Yun, Jun-Hyuk Kim. ✉e-mail: yongtae@postech.ac.kr

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Supplementary Information

Selective Electrocatalysis Imparted by Metal-Insulator Transition for Durability

Enhancement of Automotive Fuel Cells

Sang-Mun Jung1†, Su-Won Yun2†, Jun-Hyuk Kim2†, Sang-Hoon You1, Jinheon Park1, Seonggyu

Lee3, Seo Hyoung Chang4, Seung Chul Chae5, Sang Hoon Joo6, Yousung Jung3, Jinwoo Lee3,

Junwoo Son1, Joshua Snyder7, Vojislav Stamenkovic8, Nenad M. Markovic8 and Yong-Tae Kim1★

1Department of Materials Science and Engineering, Pohang University of Science and

Technology, Gyeongbuk 37673, Republic of Korea

2Department of Energy System, Pusan National University, Busan 46241, Republic of Korea

3Department of Chemical Engineering, Korea Advanced Institute of Science and Technology,

Daejeon 34141, Republic of Korea

4Department of Physics, Chung-Ang University, Seoul 06974, Republic of Korea

5Department of Physics Education, Seoul National University, Seoul 08826, Republic of Korea

6Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan

44919, Republic of Korea

7Department of Chemical and Biological Engineering, Drexel University, PA 19104, USA

8Materials Science Division, Argonne National Laboratory, IL 60439, USA

†These author contributed equally to this work

*e-mail: yongtae@postech.ac.kr

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Supplementary Figures

Supplementary Fig. 1┃Electrochemical measurement of HOR selective catalysts. a, CV of GC

and HxWO3/W, as a function oxidation cycles, in Ar purged 0.1 M HClO4 at 50 mV s-1 and b, CV

of Pt sputtered GC and HxWO3/W, as a function of oxidation cycles, in Ar purged 0.1 M HClO4

at 50 mV s-1. c, ORR polarization curves for Pt/GC, Pt/HxWO3/W at 1600rpm in O2 saturated 0.1

M HClO4 at 10mV s-1. d, HOR polarization curves at 1600rpm in H2 saturated 0.1 M HClO4 at

10mV s-1.

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Supplementary Fig. 2┃X-ray diffraction patterns of W and WO3/W on Si

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Supplementary Fig. 3┃a, b, c, d Auger electron spectroscopy measurements of the HxWO3

(x=0) layer thickness after 0 (a), 100 (b), 500 (c), and 1000 oxidation cycles(d).

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Supplementary Fig. 4┃a, Focused Ion Beam(FIB) cross-sectional HAADF-STEM image of W

pellet with Pt sputtering and b, elemental mapping of STEM image showing Pt(green) and W(red).

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Supplementary Fig. 5┃a, b, High resolution TEM images of C coated Cu grid with an

equivalent of 1 nm of sputtered Pt. c, HAADF image for EDS mapping and d, elemental mapping

image indicating the presence of Pt.

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Supplementary Fig. 6┃a, High resolution TEM b, EELS image of C coated Cu grid with an

equivalent of 1 nm of sputtered Pt. The tiny points contrasted with the carbon background is the

sputtered platinum.

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Supplementary Fig. 7┃Electrochemical measurements of HxWO3/W catalysts. a, CV of Pt/GC

and Pt-free m-HxWO3. b, ORR (solid line) and HOR (dashed line) polarization curves at 1600rpm.

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Supplementary Fig. 8┃ Rotating ring-disk electrode (RRDE) measurements. a, The disk

current densities and b, the ring current densities during the RRDE measurements of the ORR on

the Pt/GC and Pt/HxWO3/W catalysts with a rotation rate of 1600 rpm in a 0.1 M HClO4 solution.

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Supplementary Fig. 9┃X-ray diffraction (XRD) results on 60-nm-thick WO3 film grown on

(101̅2) Al2O3 (r-plane) single-crystal substrate by pulsed laser deposition.

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Supplementary Fig. 10┃In-situ resistance modulation measurement under repeated

atmosphere change between H2 and O2.

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Supplementary Fig. 11┃Ion trapping effect. a, b Cyclic voltammetry data at different cycle

number: well-ordered WO3 thin film on r-Al2O3 single-crystal (a). Electrochemically prepared

porous WO3 film on polycrystalline W pellet (b). c, d SEM images: before CV (c) and after CV

for well-ordered WO3 thin film (d). before CV (e) and after CV for polycrystalline W pellet (f). g,

h Scheme of ion trapping effect: In the case of well-ordered WO3 thin film (g) and in the case of

electrochemically prepared porous WO3 film (f).

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Supplementary Fig. 12┃ a, b, c, d The cross-sectional SEM images. (scale bar, 200nm) before

CV (a) and after CV 1000cycles for well-ordered WO3 thin film (b). before CV (c) and after CV

1000cycles for the sputtered W film prepared by RF magnetron sputtering on Si wafer (d).

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Supplementary Fig. 13┃Cyclic voltammetry data for WO3 nanoparticle at different cycle

number for a scan rate 50mV s-1.

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Supplementary Fig. 14┃ a, b In-situ resistance modulation measurement.

Characteristic of electrochemically formed porous WO3 film (a) and well-ordered WO3

film in hydrogen/Air atmosphere (b). In-situ resistance modulation measurement under

repeated atmosphere change between hydrogen and air at 60C.

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Supplementary Fig. 15┃a, b Ex-situ XPS spectra of Pt/HxWO3/W. Pt 4f at 0.05V (a) and Pt 4f

at 1.2V (b).

The potentials of both 0.05 and 1.2 V are simply the constant hold potential applied after

formation of the oxide layer and just before removal from the electrochemical cell and transfer to

the XPS chamber. At a hold potential of 0.05 V vs. RHE, the HxWO3 is in the proton intercalated

form and at a hold potential of 1.2 V vs. RHE, the support is in the proton deficient WO3 form.

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Supplementary Fig. 16┃Solid state proton nuclear magnetic resonance (1H NMR) spectra: WO3

exposed to 0.05V(Red), WO3 do not exposed to 0.05V(black), respectively.

There is a clear signal at 6.88ppm corresponding to intercalated proton in the lattice of WO3 only

for the samples exposed to 0.05 V (vs. RHE), providing a direct evidence on the intercalated

hydrogen as an origin of the MIT phenomenon in Pt/m-HxWO3.

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Supplementary Fig. 17┃UV-vis spectra of WO3 film on ITO glass.

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Supplementary Fig. 18┃a, b Bulk structure of tungsten oxide from DFT calculation. HW8O24

(a) and W8O24 (b).

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Supplementary Fig. 19┃a, b, c, d Electronic density of states (DOS) for HxWO3 (x = 0) (a) and

HxWO3 (x = 0.1) (b). The d-band center for Pt/HxWO3 (x = 0) (c) and Pt/HxWO3 (x = 0.1) (d).

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Supplementary Fig. 20┃CV of m-HxWO3 for calculation of intercalated electrical charge

of protons in Ar saturated 0.1 M HClO4 at 50 mVs-1

In this study, the electrical charge was initially calculated based on weight of mesoporous

WO3 sample. Here’s the calculation details. Firstly, the coulombic charge for proton

intercalation (QH) was calculated by integrating the proton intercalation area of CV (< 0.4 V

vs. RHE), and the mole of intercalated protons (η) could be derived using the Faraday

constant (F) (Supplementary equation 1).

η =𝑄𝐻

𝐹 (1)

where the amount of m-HxWO3 loading is 380 μg cm-2, Electrode area is 0.19625 cm2, and

Faraday constant is 96,485 C mol-1.

ηWO3 = 380 μ𝑔 𝑐𝑚−２

231.84∗10−６ 𝜇𝑔 𝑚𝑜𝑙−1 = 1.639 ∙ 10-6 mol cm-2

QH=0.16364ｘ1000 (mA mV)

50 mVs−1 × 0.19625 cm2 = 16.667 mC cm−2

ηH=𝑄𝐻

𝐹=1.728 × 10-7 mol cm-2

ηH : ηWO3 = 0.105 : 1

Therefore, x = 0.105

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Supplementary Fig. 21┃a, b Optimized geometries from DFT calculation. Pt/HWO3 (a) and

Pt/WO3 (b).

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Supplementary Fig. 22┃Electrochemical measurements of Pt/GC and Pt/HxWO3/W without

iR correction verify to the effect of iR drop.

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Supplementary Fig. 23┃a, b SEM image of m-HxWO3 (x=0). Magnified 100K (a) and 200K

(b).

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Supplementary Fig. 24┃a, HR-TEM image of Pt/m-HxWO3 (x=0), b, HAADF-STEM image

for EDS mapping. c, d elemental mapping image of Pt (c) and W (d).

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Supplementary Fig. 25 | High Resolution TEM images and particle size distributions. a-f, Pt/m-HxWO3 and g-l, Pt/C nanoparticles. (111) (Yellow), (100)

(Red), (110) (Blue), respectively.

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Supplementary Fig. 26┃Comparison of electrochemical behaviors for high loading Pt on

mesoporous HxWO3. a, CV of Pt/C (black), Pt/m-HxWO3 (5 wt%, red), Pt/m-HxWO3 (30 wt%,

blue) and Pt/m-HxWO3 (50 wt%, green) in saturated Ar 0.1 M HClO4. b, HOR and ORR

polarization curves at 1600 rpm in H2 (HOR)/O2 (ORR) saturated 0.1 M HClO4 at 10 mV s-1.

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Supplementary Fig. 27┃Comparison of electrochemical behaviors (CV, HOR and ORR) for

HOR selective catalysts using MIT (Pt/m-HxWO3), Resistivity (Pt/m-SnO2) and CME

(Pt/C_dodecanthiol) methods. a, CV of Pt/C (black), Pt/m-HxWO3 (red), Pt/m-SnO2 (blue) and

Pt/C_dodecanethiol (green) in saturated Ar 0.1 M HClO4. b, HOR and ORR polarization curves

at 1600 rpm in H2 (HOR)/O2 (ORR) saturated 0.1 M HClO4 at 10 mV s-1.

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Supplementary Fig. 28┃Single cell performance of Pt/m-SnO2 (5 wt%) and Pt/m-SnO2 (20

wt%).

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Supplementary Fig. 29┃Scheme of testing protocol for simulated SU/SD events. a, Normal

operation b, SU/SD events.

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Supplementary Fig. 30┃Measurement of single cell performance and durability. Single cell

PEMFC polarization curves for a, Pt/m-SnO2 anode : Pt/C cathode MEAs before and after SU/SD

protocols. b, Direct measurement of cell and electrode potential with DHE for Pt/m-SnO2 anode :

Pt/C cathode.

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Supplementary Table

Supplementary Table. 1┃Conductivity of HxWO3 and carbon.

Catalyst Resistivity

(Ω·cm)

Conductivity

(Ω-1·cm-1)

HxWO3 110 X 10-5 0.011 X 105

Carbon 92 X 10-5 0.009 X 105

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Supplementary Table. 2┃W4f core level distiribution in proton interclation/deintercalation of

HxWO3/W

Catalyst Binding

energy(eV) FWHM Height Area

Relative

intensity

(%)

HxWO3/W

@ 0.05 V

W6+ 4f5/2 36.06 1.16 9408 11616.8 49.3

W6+ 4f7/2 38.21 1.24 6160.7 8131.8 34.5

W5+ 4f5/2 34.85 0.86 2105 1927 8.2

W5+ 4f7/2 37.00 0.75 2380 1900 8.1

HxWO3/W

@ 1.2 V

W6+ 4f5/2 35.99 1.09 9580.7 11956.8 54.8

W6+ 4f7/2 38.14 1.17 6281.8 8967.6 41.1

W5+ 4f5/2 34.67 0.66 700 488.1 2.2

W5+ 4f7/2 36.87 0.62 626.3 414.9 1.9

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Supplementary Table. 3┃ECSA of Pt/C and Pt/m-HxWO3 (5 wt%).

Catalyst [Pt]

(μg cm-2)

QHupd

(mC cm-2)

ECSA

(m2 g-1)

Pt/C 20 2.9106 69.017

Pt/m-HxWO3 20 2.0230 48.333