O2Reduction at a DMSOCu111 Model Battery Interface Angelika DemlingyzSarah B. KingyxPhilip Shushkovkand Julia St ahleryz

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O2Reduction at a DMSO/Cu(111) Model

Battery Interface

Angelika Demling,†,‡Sarah B. King,∗,†,§Philip Shushkov,¶,kand Julia St¨ahler∗,†,‡

†Department of Physical Chemistry, Fritz Haber Institute of the Max Planck Society, 14195

Berlin, Germany

‡Humboldt-Universit¨at zu Berlin, Institut f¨ur Chemie, 12489 Berlin, Germany

¶Department of Chemistry, Tufts University, Somerville, Massachusetts 02155, USA

§Present address: Department of Chemistry and James Franck Institute, University of

Chicago, Chicago. Illinois 60637, USA.

kPresent address: Department of Chemistry, Indiana University, Bloomington, India

47405, USA

E-mail: sbking@uchicago.edu; staehler@fhi-berlin.mpg.de

Abstract

In order to develop a better understanding of

electrochemical O2reduction in non-aqueous

solvents, we apply two-photon photoelectron

spectroscopy to probe the dynamics of O2re-

duction at a DMSO/Cu(111) model battery in-

terface. By analyzing the temporal evolution of

the photoemission signal, we observe the forma-

tion of O –

2from a trapped electron state at the

DMSO/vacuum interface. We ﬁnd the vertical

binding energy of O –

2to be 3.80 ±0.05 eV,

in good agreement with previous results from

electrochemical measurements, but with im-

proved accuracy, potentially serving as a basis

for future calculations on the kinetics of elec-

tron transfer at electrode interfaces. Modelling

the O2diﬀusion through the DMSO layer en-

ables us to quantify the activation energy of

diﬀusion (31 ±6 meV), the diﬀusion constant

(1 ±1·10−8cm2/s), and the reaction quenching

distance for electron transfer to O2in DMSO

(12.4 ±0.4 ˚

A), a critical value for evaluating

possible mechanisms for electrochemical side re-

actions. These results ultimately will inform

the development and optimization of metal-air

batteries in non-aqueous solvents.

Introduction

O2reduction reactions (ORR) are one half

of the principle reactions of metal-air batter-

ies, which promise extraordinarily high energy

storage density from earth abundant materials.

However, numerous issues hinder large scale

industrial use, from competing side-reactions

at the oxygen reduction electrode that form

insulating, insoluble, O2-containing salts, to

high overpotentials for oxygen reduction due to

sluggish oxygen reduction kinetics.1Develop-

ing the precise solvent and electrolyte systems

to promote desired reactivity and hinder side-

reactions is crucial for developing metal-air bat-

tery technology, but often relies upon trial and

error methods because we have a limited un-

derstanding of the fundamental distances and

reaction energies relevant for oxygen reactivity

at electrode interfaces.

Dimethyl sulfoxide (DMSO) is a non-aqueous

solvent that has attracted attention as a sol-

vent in lithium-,2–5 zinc-,6and sodium-air bat-

teries,7because of its role in modifying the

energies of critical intermediates of the ORR.

It is proposed that DMSO stabilizes O –

2at

a distance far enough from the electrode to

prevent the formation of O 2 –

2and the insu-

arXiv:2210.13528v2 [cond-mat.mtrl-sci] 11 Jan 2023

lating, insoluble, O2-containing electrode pas-

sivation side-products.5,8,9 Molecular dynamics

(MD) simulations predict that, immediately af-

ter the ORR, O−

2is pushed approximately 10 ˚

away from the electrode at negative cathode po-

tentials making a second electron transfer un-

likely.10 Determining the thickness of DMSO

that prevents electron transfer to O2, and there-

fore how far O –

2needs to be away from an

electrode to not react, is critical to designing

DMSO-based electrolytes for metal-air batter-

ies, but is currently unknown and a focus of this

work.

The Marcus theory of electron transfer has

been used to predict the kinetics of electron

transfer at electrode interfaces.11–13 However,

successful application of the theory requires ac-

curate information about the energies of donor

and acceptor species both before and after elec-

tron transfer, such as vertical binding ener-

gies and adiabatic electron aﬃnities, to calcu-

late the reorganization energy and energy of re-

action that determines the interfacial reaction

rate. The relevant energies for O2and O –

2in

DMSO have been estimated from the O2/O –

redox potential in DMSO measured with cyclic

voltammetry (CV). The O2/O –

2redox poten-

tial ranges from -0.73 V in a 0.1 M (Et)4NClO4-

DMSO solution with respect to a standard

calomel electrode (SCE)14 to 2.7 V in a 0.1 M

TBAClO4-DMSO solution versus Li/Li+using

in situ surface enhanced Raman spectroscopy

(SERS) measurements.15 These redox poten-

tials can be compared to the vacuum level of

the working electrode through the standard hy-

drogen electrode and its vacuum level (details

in the Supporting Information),16–18 but result

in a fairly wide range of energies as shown in

Figure 1(a) (ﬁrst two columns). These factors

make the accurate determination of the vertical

binding energy (VBE) of O –

2in DMSO chal-

lenging and prevent accurate modeling of elec-

tron transfer reactions at electrode surfaces.

Owing to their importance in a variety of

research ﬁelds, electron solvation processes

have been subject of numerous studies in the

ultrafast time domain applying optical and

terahertz spectroscopy as well as photoemis-

sion.19–24 Two-photon photoemission (2PPE)

spectroscopy of solvent molecular layers has

been used to probe the energies, solvation dy-

namics, and reactivity of electronic states in

molecular layers ranging from H2O, NH3, and

organic solvents like DMSO, to liquid crys-

tals.25–33 By using ultrafast laser pulses to cre-

ate excited electrons above the Fermi level of

a metal substrate, non-equilibrium electrons

can be injected into adsorbed molecular lay-

ers and probed using a second laser pulse de-

layed by femtoseconds to microseconds. Us-

ing this strategy, 2PPE has been able to ob-

serve the 10s-100s of femtoseconds formation of

small polarons in DMSO. In a previous publica-

tion, we discuss the electron transfer from these

electronic states located on two wetting mono-

layers of DMSO to much longer-living elec-

tronic states located on thicker DMSO islands

with lifetimes on the order of seconds.31 A car-

toon of the transfer process is displayed in Fig-

ure 1(b). Furthermore, 2PPE is capable of

determining the population dynamics of such

long-lived states by applying pump-wait-probe

experiments (cf. Supporting Information) or

measuring the repetition rate dependence of the

respective feature.34–36Even chemical reactions

involving long-lived “trapped” electrons can be

detected in 2PPE experiments. For example,

on water-ice surfaces, it was shown that trapped

electrons can react with co-adsorbed molecules,

breaking H−OH and Cl−CCl2F bonds and gen-

erating highly reactive hydroxide and chloride

ions.25,26,37

In this paper, we use monochromatic 2PPE

of O2adsorbed on DMSO molecular layers on

Cu(111) to probe the electronic states of DMSO

and O2at a model electrode interface. We show

that trapped electrons of DMSO (P2shown in

Fig. 1(b)) serve as a precursor for O –

2forma-

tion. We measure the VBE of O –

2to be 3.80

±0.05 eV.

By investigating the electron transfer dynam-

ics from the trapped electron state in DMSO

to O2, i.e. the ﬁrst ORR, along with modelling

O2diﬀusion into the DMSO adlayer, we can ob-

serve the distance dependence of O2reduction

and identify the reaction quenching distance for

electron transfer in DMSO. Through these ex-

periments we have determined two components

Figure 1: (a) Energy level alignment of O−

2relative to Evac based on electrochemical14–16,18 (red-

bounded) and photoemission experiments (red). (b) Illustration of photoinduced polaron formation

(process 1) at the surface of the two wetting monolayers of DMSO and subsequent electron trapping

at the multilayer/vacuum interface (process 2). P1and P2denote the respective spectral signatures.

critical for accurate models of electrochemical

systems with DMSO and O2, the VBE of O−

and its formation distance, which will direct re-

search eﬀorts to prevent electrode passivation

by unwanted oxygen reduction pathways.

Methods

The Cu(111) crystal is prepared by re-

peated cycles of sputtering at 0.75 kV with

1.5 ·10−6mbar Ar+ions for 10 min followed

by annealing at 800 K for 45 min. The surface

cleanliness and order is veriﬁed by LEED, work

function (Φ) measurements, and the width and

intensity of the surface state characteristic for

Cu(111) in 2PPE spectra.38 The ≥99.9% an-

hydrous DMSO (Sigma Aldrich) is attached to

the gas manifold of the ultrahigh vacuum sys-

tem in an Argon atmosphere and cleaned by

numerous freeze-pump-thaw cycles. Its cleanli-

ness is conﬁrmed by residual gas analysis. The

DMSO molecules are deposited onto the cop-

per substrate through a pinhole doser with a

diameter of 5 µm and a backing pressure of

6·10−1mbar. First, molecules are deposited

for 210 s with the Cu crystal temperature held

at 200 K. Afterwards the sample is annealed for

ten minutes at 210 K before further molecules

are deposited at 150 K for 135 s, followed by

a second annealing at 180 K for 10 minutes.

As discussed previously,31 using this method

a reproducible adsorption of two crystalline

DMSO monolayers partially covered with mul-

tilayer islands is achieved and the nominal layer

thickness is veriﬁed with thermal desorption

spectroscopy. More information about sam-

ple preparation and characterization of DMSO

adlayers can be found in a previous publica-

tion and the corresponding Supporting Infor-

mation.31 O2molecules are adsorbed onto the

DMSO/Cu(111) sample by background dosing

at 46 K. At this temperature only a monolayer

of O2can be physisorbed on the surface.39 All

referenced temperatures are measured using a

K-type thermocouple inside the copper crystal.

The laser system is a Light Conversion Pharos

pump laser combined with a non-linear optical

parametric ampliﬁer (Orpheus 2H) operating at

200 kHz. This system delivers ultrashort laser

pulses tunable from the visible to near UV. In

the described experiments, photon energies be-

tween 2.9 eV and 3.2 eV are used, with pulse

durations of approximately 100 fs.

Monochromatic 2PPE is a surface-sensitive

technique that can determine the energies of

both occupied and normally unoccupied elec-

tronic states with respect to the Fermi and vac-

uum level of the sample. The working principle

is depicted in Figure 2(a). In 2PPE, the ab-

sorption of two photons ionizes the sample and

generates photoelectrons with ﬁnite kinetic en-

ergies with respect to the vacuum level, Evac.

The kinetic energies, Ekin, of the photoemit-

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摘要：

O2ReductionataDMSO/Cu(111)ModelBatteryInterfaceAngelikaDemling,y,zSarahB.King,,y,xPhilipShushkov,{,kandJuliaStahler,y,zyDepartmentofPhysicalChemistry,FritzHaberInstituteoftheMaxPlanckSociety,14195Berlin,GermanyzHumboldt-UniversitatzuBerlin,InstitutfurChemie,12489Berlin,Germany{DepartmentofChemi...

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O2Reduction at a DMSOCu111 Model Battery Interface Angelika DemlingyzSarah B. KingyxPhilip Shushkovkand Julia St ahleryz

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