Supporting Information Bromide Protection of the Pt(hkl) crystals and crystal quality

Supporting Information

Bromide Protection of the Pt(hkl) crystals and crystal quality

Figure S 1: Left: CV showing a bromide protected Pt(111) single crystal (red) and the electrochemical stripping of bromide in 0.1 M H2SO4. After the stripping the well known Pt(111) CV is observed (black traced CV). Right figure: Pt (100) CV after bromide stripping.

DEMS Measurement for the ORR on an Au electrode

-40 -20 0 20 40

-1.6 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2

-120 -100 -80 -60 -40 -20 0 20 IF/µAI32/pA

E/V vs. Ag/Ag⁺ (a)

-40 -20 0

-1.8 -1.6 -1.4 -1.2 -1.0

1.0 1.2 1.4 1.6 1.8 2.0 IF/µAe- pro O2

E/ V vs. Ag/Ag⁺ (b)

Figure S 2: a) CV and MSCV m/z = 32 of 0.4 M NaClO4/DMSO, b) electronnumber during ORR @ Au electrode (A = 0.283 cm²) v =10 mV s^-1; flowrate = 5 µL s^-1; electrolyte saturated with a mixture of 20:80 O2:Ar.

The shoulder in the CV in the ORR range corresponds to the diffusion limited 1 e^- reduction to the soluble superoxide. The peak is due to the 2 e^- reduction to peroxide, which is deposited on the surface. Integration of this latter potential range results in a charge of around 1300 µC cm^-2, or 650 µC cm^-2 after dividing by the roughness factor of 2. This corresponds to 1.5

monolayers of peroxide (390 µC cm^-2 per monolayer and hypothetically assuming 1 to 1 adsorption of 2−¿

O₂^¿ per metal surface atom).

CV study on the peak shape of the ORR on Pt and Au electrodes

Figure S 3: CV study in a 0.1 M NaClO4in DMSO electrolyte on (a) a Pt working electrode and (b) an Au working electrode. The sweep rate in the CVs was varied. The electrolyte was saturated with a mixture of 20:80 O2:Ar.

In Figure S 3 CVs are shown on a (a) Pt electrode and (b) Au Electrode without rotation in a 0.1 M NaClO4/DMSO electrolyte, which is saturated with an oxygen concentration of 20:80 O2:Ar. The sweep rate is increased in each cycle from 10 mV/s up to 100 mV/s in a potential the range between -0.9 V and -1.6 V vs. Ag/Ag⁺.

Sweep rate dependence of the first ORR Peak

Figure S 4: Randles-Sevcik-Plot for at Pt- and Au-Electrode for the reduction Peak C1 shown in Figure S1 assuming a reversible reaction. The strong deviation of the obtained diffusion constants ((on Pt:

D(O2) = 19.5⋅10^-6 cm² s-¹; on Au: D(O2) = 14.5⋅10^-6 cm² s^-1) from the literature value ([1]) and also the slight deviation from linearity is due to the fact that the reaction is neither completely reversible nor totally irreversible ([2]). i.e. at the borderline between both limiting cases.

RRDE Measurements in Na⁺ containing DMSO saturated with pure oxygen

Figure S 5: RRDE studies in 0.1 M NaClO4/DMSO with different rotation rates on (a) Pt and (b) Au (A = 0.164 cm²). The electrolyte was saturated with 100 % O2 . The sweep rate was v = 10 mV s^-1 and the ring potential was held at +0.3 V vs Ag⁺|Ag during all experiments. Disk-current is normalized to the geometrical surface area and ring-current to the theoretical collection efficiency (N0) and geometrical surface area of the disk electrode.

-1,6 -1,5 -1,4 -1,3 -1,2 -1,1 -1,0

0,6 0,7 0,8 0,9 1,0 1,1

4 9 16 25 36 4 repeat

Share of Superoxide X

E/V vs. Ag/Ag⁺

f / Hz

@ Pt 100% O₂ (a)

-1,6 -1,5 -1,4 -1,3 -1,2 -1,1 -1,0

0,6 0,7 0,8 0,9 1,0 1,1

4 9 16 25 36 4 repeat

Share of Superoxide X

E/V vs. Ag/Ag⁺

f / Hz

@ Au 100% O₂ (b)

Figure S 6: Calculated share of superoxide for the measurements shown in Figure S 5.

Kinetic analysis from RRDE measurements:

Figure S 7: Levich-Koutecký extrapolations for the Au electrode at different disk potentials with 20/80 O2:Ar saturation of 0.1 M NaClO4/DMSO.

Figure S 8: Determination of the half-wave Potential at Pt electrode using the currents obtained at a rotation rate of 4 Hz.

Kinetic Analysis from CV data with the method from Lavagnini and Nicholson:

This method is based on the extraction of values from the working function Ψ with the help of the experimentally determined peak separation ∆E = A1-C1. The unit-less Ψ - function is created with the following equation:

ψ=(−0.6288+0.0021⋅ΔE)

(1−0.017⋅ΔE) ⁽¹⁾

In equation Error: Reference source not found is ∆E the peak separation in mV. A plot of the working curve Ψ as well as the extracted Ψ values for the experimentally determined ∆E values is shown in Figure S 7(a). With a plot of Ψ vs. v^-1/2 we get the heterogeneous rate constant out of the slope of this plot and the following equation.

Ψ=k₀(D_O D_R)

α

2

√

^√

DO is the diffusion coefficient of oxidized species (O2 in 0.1 M NaClO4 in DMSO; D = 23.8

⋅ 10^-6 cm²/s [1]), DR of reduced species (O2-, D = 2.31*10^-6 cm²/s (0.1 M NaClO4/DMSO)), ν is the sweep rate, α transfer coefficient ( α=0.5¿ , the other parameters have the common meaning. (We determined the diffusion coefficient of superoxide by performing potential jump experiments with the RRDE setup at various rotation rates. (Fig. S11 and [3]). The plot of Ψ vs. v^-1/2 is shown in Figure S 7(b). There we observe a linear trend, which allows us the determination of ^k0 . Only for the lowest sweep rate (black cycle v = 10 mV/s, v^-0,5 = 0.3162 (s/mV)^0.5), the measurements at the Pt electrode show a deviation from the linear trend, and this data point was not included in the evaluation.

Figure S 9: (a) Plot of the working curve Ψ. Also shown are the extracted values of Ψ for the experimentally determined values for the measurement on the Pt electrode (black points) and the Au electrode (red points). The corresponding CVs are shown in Figure S 3. (b) Plot of the extracted Ψ

for which the denominator in eq.1 becomes zero and the resolution of the measured data points was not sufficient for an exact determination at such low sweep rates.

Figure S 10: Graphical representation of the evaluation of the equilibrium potential a) Pt electrode b) Au electrode.

Determining the diffusion coefficient of superoxide in Na⁺ containing DMSO

The diffusion coefficient of superoxide was determined by performing potential jump experiments with the RRDE setup. There the ring is held a constant potential, at which the reoxidation of superoxide can occur. The disc potential is abrupt stepped into the superoxide formation. To determine the diffusion coefficient of superoxide the transfer time t_s between the disc and the ring electrode is determined using the ring current transient. With the help of the following equation the diffusion coefficient of superoxide can be determined:

t_s=3.58

(

)

[

⁽

⁾ ]

Herein v is the kinematic viscosity of the electrolyte, D the diffusion coefficient of superoxide, f the rotation frequency of the electrode, ^r2 the outer gapradius and ^r1

the dic radius.

The step program as well as the disk and ring currents are shown in Figure S8 a, b and c. The determination of the diffusion coefficient of the superoxide species, is carried out of a shielding experiment: A potential of 0.3 V is applied to the ring which leads to the oxidation of superoxide. The disk potential is stepped from -0.5 V where no reaction occurs, to -1.2 V where the superoxide formation and then to the potential where the reoxidation at the disk takes place. From the time delay between disk and ring current, the diffusion coefficient of superoxide can be determined with the equation above. The determination of ts is elucidated in Figure S 8c. The blue dashed line represents a tangent of the turning point of the ring transient and the intercept of the time-axis with the tangent gives the value for ts at a specific rotation rate.

0 2 4 6 0

2 4 -40 -20 0

I

/µ A

t/sec

Au-Ring

E(Ring) = 0.3V

T_S

c

I

/µ A

Au-Disc

b

Figure S 11: (a) Exemplary potential jump experiment with the RRDE. (b) Disk current on the Au electrode after the jump. (c) Corresponding ring current. Blue line shows the determination of transfer time of superoxide.

CV study in LiClO4/DMSO electrolyte with 20 % O2 on Au

Figure S 12: CV study in a 0.1 M LiClO4 in DMSO electrolyte saturated with 20 % oxygen on an Au working electrode at a constant sweep rate of 10 mV s^-1. The lower potential was reduced in 100 mV steps in every cycle.

Table S 1: Charges under Rotation of the ORR and OER for peroxide formation and oxidation and corresponding amount in ML (see Figure 11).

Pt (RF =

1.5)

Au (RF = 1.6)

Pt (Rf = 1.5)

Au (Rf = 1.6) QORR/

µC cm^-2

QORR/ µC cm^-2

ML

@ Pt

ML

@ Au

QOER/ µC cm^-2

QOER/ µC cm^-2

ML @ Pt

ML

@ Au

1:0 -474 -1265 0.8 2.0 30 89 0.06 0.1

1000:1 -921 -1447 1.5 2.3 901 1155 1.4 1.9

62:1 -503 -1134 0.8 1.8 914 1098 1.5 1.8

5:1 -836. -1226 1.3 2.0 718 757 1.1 1.2

0:1 -1153 -1583 1.8 2.5 658 1235 1.1 2.0

average -778 -1331 1.24 2.12

Average.for Li⁺ cont’g el.

644 1061 1.3 1.7

[1] P.H. Reinsberg, P.P. Bawol, E. Thome and H. Baltruschat, Anal. Chem., 90 (2018) 14150.

[2] A.J. Bard and L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, John Wiley & Sons Inc., New York, Weinheim, 2001.

[3] P.H. Reinsberg, A. Koellisch, P.P. Bawol and H. Baltruschat, Phys. Chem. Chem.

Phys., 21 (2019) 4286.