1

Supporting Information

Understanding the Interface between Electrode and

Electrolyte: A Hybrid Organic/Inorganic Design

for Fast Ion Conductivity

Dong Young Chung1,2

, Young-Hoon Chung3, Sungmin Kim

4, Ju Wan Lim

2,

Kyung Jae Lee1,2

, Namgee Jung5, Hyeyoung Shin

4, Ok-Hee Kim

6, Hyungjun

Kim4, Sung Jong Yoo

3,* and Yung-Eun Sung

1,2,*

2

1. Modified TLM for EIS analysis

Figure S1. Equivalent circuit for modified transmission line model under H2/N2 non-faradaic

condition

Figure S2. Cyclic voltammetry curves for commercial Pt/C MEA H2/N2 anode and cathode,

respectively.

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2. Detail experimental conditions of Figure 2 and Figure 3 results

Table S1. Detail experimental conditions of Figure 2 and 3 results.

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3. TLM results of fully humidified and different temperature conditions

Figure S3. Electrochemical impedance spectroscopy (EIS) Nyquist plot of data by altering

cell temperature and relative humidity

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4. Molecular dynamics (MD) method and details

Computational Details

Molecular dynamics (MD) simulations were performed to understand the effect of 3-MPA

molecules adsorbed on the electrode system using the LAMMPS software program. 1

To achieve full equilibration in the polymeric phase, we used the scaled-effective-solvent

(SES) method where all non-bonded interactions are effectively scaled down over the course

of the rotation-configurational space sampling 2.To attain insights about the catalyst-polymer

membrane interface at the operational conditions, we compared cases when our model system

for Pt electrode is uncharged and that is charged by +5.

We considered bare and 3-MPA modified Pt systems where the coverage of the 3-MPA

molecules is chosen to be ~25 % in aqueous solutions. When the electrode was charged by +5,

we further included 5 Cl- ions to satisfy the charge balance of the simulation cell. Our Pt

electrode is modelled by a three-layer Pt (111) slab with (8 × 9) periodicity.

Figure S4. Chemical structures of Nafion and chemical species used for determining charges

of Nafion. (a) Nafion. We used a single Nafion chain having equivalent weight of 1157 and

composition of n=10, x=7, y=1, and z=1. As we highlight the Fig. S1a, to determine the

partial charges using density functional theory calculations, we considered three segments of

the Nafion chain; (b) perfluorohexane, (c) 1,1,1,2,2,3,3,4,5,5,6,6,6-tridecafluoro-4-

(perfluoroethoxy)hexane, and (d) 2,2'-oxybis(1,1,2,2-tetrafluoroethanesulfonate), which

represent the part I, part II, and part III of Nafion chain, respectively.

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The polymer membrane is modeled using a single chain of Nafion (equivalent weight of

1157, Figure S4a) and with 11 weight percent of water (70 H2O and 10 H3O+ ions) is

immersed in the polymer membrane. To describe interatomic potential, we used DREIDING

generic force field potential. 3 Using density functional theory (DFT) calculations and

Mulliken population analysis 4, we determined the partial charges of Nafion and H3O+ ion

(B3LYP 5-6 / 6-31G** 7 level using Jaguar 7.9 Program 8. Figure S4 shows the segments of

Nation chain used to determine the partial charge distribution using DFT calculations, and

partial charges are shown in Table S2. To model the partial charge distribution of the Pt

electrode surface, we used the charge equilibrium (QEq) method 9. Water molecules were

described using the flexible three-centered (F3C) water model 10.

Our overall MD simulation procedures were summarized as follows:

1. Scaled effective solvent (SES) procedure 2 to predict the equilibrium structure of

Nafion.

2. Inclusion of H2O, H3O+, and Cl- on the Nafion system using grand canonical Monte

Carlo (GCMC) simulations with fixed final number of the species to introduce them

into favorable positions.

3. Canonical ensemble (NVT) dynamics to heat up the system from 10 K to 343 K for 10

ps.

4. Annealing dynamics changing the temperature from 343 K to 413K for 5 cycles.

5. Isothermal-isobaric ensemble (NPT) dynamics along with the z-dimension at 1 atm

and 343K for 4 – 45 ns until the volume of the complex system reach to the

equilibrium state.

6. NVT dynamics at 343 K with the volume determined from the NPT dynamics step for

5 – 30 ns until the systems reach to the equilibrium.

Here, the simulation time step is set as 1 fs and all Pt atoms are fixed for the whole procedure.

All analysis is based on the final 5 ns NVT trajectories.

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Table S2. Partial charges of a Nafion (Figure S4a) determined from Mulliken charges using

DFT calculations with B3LYP/6-31G** level for perfluorohexane (Figure S4b, for part Ⅰ),

1,1,1,2,2,3,3,4,5,5,6,6,6-tridecafluoro-4-(perfluoroethoxy)hexane (Figure S4c, for part Ⅱ),

and 2,2'-oxybis(1,1,2,2-tetrafluoroethanesulfonate) (Figure S4d, for part Ⅲ).

Part Atom type Charge

I

C1 0.538

F1 -0.269

C2 0.538

F2 -0.269

II

C3 0.584

F3 -0.271

C4 0.491

F4 -0.276

O5 -0.514

C6 0.749

F6 -0.254

C7 0.491

F7 -0.276

C8 0.774

F8 -0.258

III

O9 -0.514

C10 0.803

F10 -0.271

C11 0.336

F11 -0.290

S12 1.092

O12 -0.598

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Figure S5. Molar concentration of proton sources (water molecules and hydronium ions) as a

function of distance from the Pt surface along its normal direction when the electrode is (a)

uncharged and (b) charged by +5. Bare Pt electrode is shown as black, and 3-MPA modified

Pt electrode is shown as red. This shows that 3-MPA adsorbates significantly contribute to

enhance the surface wettability near the Pt surface.

Figure S6. Local densities of proton sources (water molecules and hydronium ions) near the

Pt surface (within 5 Å) are shown for bare Pt case (black) and 3-MPA modified Pt case (red),

which are obtained from the final 5 ns MD trajectories. To attain insights about the catalyst-

polymer membrane interface at the operational conditions, we compared cases when our

model system for Pt electrode is uncharged and that is charged by +5. In both cases, we find

the surface wettability dramatically increases by modifying the catalyst surface with 3-MPA,

which is expected to enhance the proton transfer kinetics.

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Figure S7. Radial distribution function, g(r), of the oxygen atoms of the proton sources

(water molecules and hydronium ions) around oxygen atoms of carboxylate groups of 3-MPA

molecules. We note that the first peak of g(r) is very sharp and it indicates that there is a

strong interaction between the proton sources and carboxylate groups of 3-MPA molecules by

forming hydrogen bonds.

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5. Analysis of faradaic EIS (Figure 6c left in MS)

To analyze the EIS data more details conducted faradaic condition, equivalent circuit fitting

was used. A general equivalent circuit contains wire inductance (Lw), constant phase element

of anode and cathode (CPE), charge transfer resistance of both electrodes (Ranode, Rcathode),

total ohmic resistance (RPEM), and Warburg element (ZWS) which is related to mass transport

resistance. EIS was conducted at 0.6V with feeding of humidified H2 and air at anode and

cathode respectively.

Figure S8. Equivalent circuit of PEMFC under the faradaic condition

EIS data is shown at Figure 6c left and fitting results are presented Table S3

Table S3. Results of the fitting with an equivalent circuit for analysis of resistance of

individual parts.

RPEM

(Ω cm2)

Rcathode

(Ω cm2)

REF Pt/C 0.078 0.18

3-MPA Modified Pt/C 0.069 0.24

Ranode was extremely low value compared to Rcathode. Hydrogen oxidation reaction resistance

in anode could be disregarded.11-12 Cathode charge transfer resistance of modified with 3-

MPA increased about 33% relative to reference one. These results were well correlated to

decrease of ECSA measured by CV. ZWO-R that is related to the mass transport issues also

increased. The reason of these results is that high current induces the more water in system.

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6. Quantitative analysis of electrochemical surface area (ECSA), EIS results

of non-faradaic condition. (Figure 6c right in MS)

Table S4. Results of the ECSA calculated by CV and fitted resistance of membrane and

proton transport resistance in catalyst layer under non-faradaic condition.

The ECSA of the cathode catalyst layer was estimated by cyclic voltammetry (CV).13 Rmem

and Rion were calculated by Figure 6c right in MS results with modified transmission line

model.

REF-Pt/C 3-MPA

modified Pt/C

ECSA(m2g-1) 35.9 29.6

Rmem (Ωcm2) 0.082 0.081

Rion (Ωcm2) 0.091 0.067

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7. References 1. Plimpton, S. Fast parallel algorithms for short-range molecular dynamics. Journal of

Computational Physics 1995, 117 (1), 1-19.

2. Shin, H.; Pascal, T. A.; Goddard, W. A.; Kim, H. Scaled effective solvent method for

predicting the equilibrium ensemble of structures with analysis of thermodynamic properties

of amorphous polyethylene glycol-water mixtures. J. Phys. Chem. B 2013, 117 (3), 916-927.

3. Mayo, S. L.; Olafson, B. D.; Goddard Iii, W. A. DREIDING: A generic force field for

molecular simulations. J. Phys. Chem. 1990, 94 (26), 8897-8909.

4. Mulliken, R. S. Electronic population analysis on LCAO-MO molecular wave

functions. I. The Journal of Chemical Physics 1955, 23 (10), 1833-1840.

5. Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. The

Journal of Chemical Physics 1993, 98 (7), 5648-5652.

6. Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti correlation-energy

formula into a functional of the electron density. Physical Review B 1988, 37 (2), 785-789.

7. Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Self-consistent molecular orbital

methods. XX. A basis set for correlated wave functions. The Journal of Chemical Physics

1980, 72 (1), 650-654.

8. Suite 2012: Jaguar version 7.9, S., LLC, New York, NY, 2012.

9. Rappé, A. K.; Goddard Iii, W. A. Charge equilibration for molecular dynamics

simulations. J. Phys. Chem. 1991, 95 (8), 3358-3363.

10. Levitt, M.; Hirshberg, M.; Sharon, R.; Laidig, K. E.; Daggett, V. Calibration and

testing of a water model for simulation of the molecular dynamics of proteins and nucleic

acids in solution. J. Phys. Chem. B 1997, 101 (25), 5051-5061.

11. Springer, T. E.; Zawodzinski, T. A.; Wilson, M. S.; Gottesfeld, S. Characterization of

polymer electrolyte fuel cells using AC impedance spectroscopy. J. Electrochem. Soc. 1996,

143 (2), 587-599.

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12. Wu, J.; Yuan, X. Z.; Wang, H.; Blanco, M.; Martin, J. J.; Zhang, J. Diagnostic tools in

PEM fuel cell research: Part I Electrochemical techniques. Int. J. Hydrogen Energy 2008, 33

(6), 1735-1746.

13. Brummer, S. B. The use of large anodic galvanostatic transients to evaluate the

maximum adsorption on platinum from formic acid solutions. J. Phys. Chem. 1965, 69 (2),

562-571.