In Situ Formed “Sn1–XInX@In1–YSnYOZ” Core@Shell Nanoparticles as Electrocatalysts for CO2 Reduction to Formate [original]

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2103601 (1 of 14)

ReseaRch aRticle

In Situ Formed “Sn1–XInX@In1–YSnYOZ” Core@Shell

Nanoparticles as Electrocatalysts for CO2 Reduction

to Formate

Laura C. Pardo Pérez, Detre Teschner, Elena Willinger, Amandine Guiet, Matthias Driess,

Peter Strasser, and Anna Fischer*

Electrochemical reduction of CO2 (CO2RR) driven by renewable energy has

gained increasing attention for sustainable production of chemicals and fuels.

Catalyst design to overcome large overpotentials and poor product selectivity

remains however challenging. Sn/SnOx and In/InOx composites have been

reported active for CO2RR with high selectivity toward formate formation. In

this work, the CO2RR activity and selectivity of metal/metal oxide composite

nanoparticles formed by in situ reduction of bimetallic amorphous SnInOx thin

films are investigated. It is shown that during CO2RR the amorphous SnInOx

pre-catalyst thin films are reduced in situ into Sn1–XInX@In1–YSnYOz core@

shell nanoparticles composed of Sn-rich SnIn alloy nanocores (with x<0.2)

surrounded by InOx-rich bimetallic InSnOx shells (with 0.3 < y<0.4 and z≈ 1).

The in situ formed particles catalyze the CO2RR to formate with high faradaic

efficiency (80%) and outstanding formate mass activity (437AgIn+Sn

−1 @

−1.0V vs RHE in 0.1m KHCO3). While extensive structural investigation during

CO2RR reveals pronounced dynamics in terms of particle size, the core@shell

structure is observed for the different electrolysis conditions essayed, with high

surface oxide contents favoring formate over hydrogen selectivity.

DOI: 10.1002/adfm.202103601

1. Introduction

The exhaustive use of carbon-based fossil fuels has led to a pro-

nounced increase of CO2 emissions over the past century.[1–3]

The environmental impact of these emissions has drawn atten-

tion to the necessity to stabilize and eventually decrease the

L. C. Pardo Pérez,[+] A. Guiet,[++] M. Driess, A. Fischer

Institute of Chemistry—Inorganic Chemistry

Technical University Berlin

Straße des 17.Juni 135, 10623 Berlin, Germany

E-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article

can be found under https://doi.org/10.1002/adfm.202103601.

L. C. Pardo Pérez, A. Fischer

Institute of Inorganic and Analytical Chemistry

University of Freiburg

Albertstraße 21, 79104 Freiburg, Germany

D. Teschner, E. Willinger

Department of Inorganic Chemistry

Fritz-Haber-Institute der Max-Planck-Gesellschaft

Faradayweg 4–6, 14195 Berlin, Germany

D. Teschner

Department of Heterogeneous Reactions

Max-Planck-Institute for Chemical Energy Conversion

Stiftstraße 34–36, 45470 Mülheim an der Ruhr, Germany

P. Strasser

Institute of Chemistry—Technical Chemistry

Technical University Berlin

Straße des 17.Juni 124, 10623 Berlin, Germany

A. Fischer

FMF—Freiburger Materialforschungszentrum

University of Freiburg

Stefan-Meier-Straße 21, 79104 Freiburg im Breisgau, Germany

CO2 atmospheric concentrations.[4–6] In

this context, the electrochemical conver-

sion of CO2 into fuels driven by a renew-

able energy input has attracted increasing

attention as a promising approach to

chemically store renewable energy in

C-based fuels. Electrochemical reduction

of CO2 allows converting CO2 in fuels

under mild chemical conditions, such as

aqueous media and ambient temperature

and pressure. However, energy efficiency

remains elusive as overpotentials as large

as 1V are required to bring the process to

a desired production rate.

The CO2 reduction reaction, abbrevi-

ated as CO2RR, is a complex process that

requires multiple electron and proton

transfer reactions which can lead to a

wide variety of products including CO,[7–14]

HCOOH,[7,10,11,13,15–17] hydrocarbons

(mainly methane and ethylene),[13,14,18–21]

or CH3OH.[22–26] In aqueous medium

CO2RR is in competition with H2 evolu-

tion (HER). Accordingly, selectivity control in CO2RR is a major

challenge. Early studies of CO2 electroreduction in aqueous

media were primarily focused on electrode materials with a

large HER overpotential such as Hg.[27–31] In addition, the pio-

neering work of Hori,[9,14] Vassiliev,[32] and further contributions

of Azuma[7] and Ito[10,13] provided a large pool of experimental

[+]Present address: Electrochemical Conversion of CO2 group,

Helmholtz-Zentrum Berlin für Materialien und Energie,

Hahn-Meitner-Platz 1, 14109 Berlin, Germany

[++]Present address: Institut des Molécules et Matériaux du Mans

(IMMM), UMR 6283 CNRS, Le Mans Université, Avenue Olivier

Messiaen, Le Mans Cedex 9 72085, France

Wiley-VCH GmbH. This is an open access article under the terms of the

Creative Commons Attribution License, which permits use, distribution and

reproduction in any medium, provided the original work is properly cited.

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2103601 (2 of 14)

results for CO2 reduction on various metal electrodes. The

metals have been empirically classified according to the major

CO2RR reduction product in aqueous media. While metals like

Pt were found to have negligible activity for CO2RR, Ag and

Au were found to favor the production of CO. Interestingly,

Cu was found to catalyze the production of higher reduction

products such as hydrocarbons. In contrast, metallic Sn and In

electrodes have been found to catalyze the conversion of CO2

preferentially to formate (HCOO−).[7,9,10,13,14,32]

Formic acid or formate salts have multiple possible appli-

cations as building blocks for re-utilization of CO2. They can

be used as direct fuel in direct formic acid fuel cells (DFAFC),

potentially offering a CO2 neutral fuel cycle, as hydrogen storage

vector or as raw material in industry.[33–35] However so far, the

reported faradaic efficiencies offered by metallic Sn[36] and In

catalysts[37] (in potential windows between −1.0V to −1.5V vs

RHE) vary significantly among reports[7,9,43–49,13,16,17,38–42] At

applied potentials around −1.0V versus RHE, Sn and In were

found to provide formate faradaic efficiencies between 63–88%

and 69–95% respectively, while at larger reductive bias, that is,

in the range of −1.3 to −1.5V versus RHE, the production of

H2 is favored, thereby decreasing the faradaic efficiency for for-

mate to only 5.2% and 33.3% for Sn and In respectively.[7]

Recent reports have pointed out that the origin of the vari-

ability of formate faradaic efficiencies reported on Sn and In

based catalysts could be associated with the native oxide layer

present at the surface of these metals. For instance, Kanan

reported that at low applied potential of −0.7V versus RHE, a

tin foil with a native oxide layer exhibits a formate faradaic effi-

ciency of about 19%, while a freshly acid-etched foil, without a

native oxide layer virtually produced no formate, as indicated

by the extremely low value of 0.3% of the formate faradaic effi-

ciency.[38] To gain further understanding on the influence of

SnOx surface oxide species on the activation of CO2 to formate,

the authors also reported the simultaneous electrodeposition

of Sn° and SnOx (SnOx/Sn°) on a titanium foil, during CO2

electrolysis experiments at −0.7V versus RHE. A CO2 conver-

sion faradaic efficiency of 85–98% (Formate 40% + CO 58%)

was thereby found. The high selectivity toward CO2 conversion

into CO and HCOO− over HER on these electrodes strongly

suggests a boost of CO2 conversion over H2 evolution due to

the presence of surface oxides (of the type SnOx). However, the

electrodeposited SnOx/Sn° composite tends to lose its conduc-

tivity upon contact with air and consequently its activity.

A study on a hierarchical dendrite Sn structure electrodepos-

ited on a tin foil[39] revealed that the high surface area structure

could deliver higher current densities and that increasing the

oxygen content in the material's surface through heat treat-

ment in air led to significant improvement in the formate

faradaic efficiency (from 32% to 55.9% at −1.06V vs RHE). A

step further in the investigation of the role of SnOx moieties

on the CO2RR electrocatalytic activity was proposed by Meyers

group,[40] who reported in situ electrochemically reduced tin

oxide nanoparticles capable of catalyzing the conversion of CO2

into formate in NaHCO3 aqueous solutions with faradaic effi-

ciencies as high as 86.2% at −1.2V versus RHE.

An analogous behavior was reported on Indium surfaces.

Bocarsly's group reported that Indium electrodes with a native

oxide layer could provide up to 75% faradaic efficiency for

formate formation at −0.9V versus RHE. However, the stability

was limited, as over 2 h of operation, the faradaic efficiency

for formate dropped below 20%; a value typically obtained for

freshly acid-etched In electrodes. Anodized In electrodes pro-

vided a stable faradaic efficiency of 35–40% over 6 h.[41] The

same group reported a comparative study of the activity of

In(OH)3, In2O3 and metallic In nanoparticles.[42] High faradaic

efficiencies (>70%) could be reached with these catalysts at

moderate applied potentials (−0.9 to −1.2V vs RHE). Interest-

ingly, In° NP's with native InOH and InO surface species

were found to provide virtually 100% faradaic efficiency toward

formate, while pure In(OH)3 and In2O3 nanoparticles reached

lower faradaic efficiencies and required larger overpotentials,

probably due to their lower conductivity, when compared to the

metallic nanoparticles.

In regard of the mechanisms of formate production on

Sn- and In-based catalysts two possible pathways have been

proposed. Correlation of experimentally observed partial

current densities and theoretical insights on intermediates

binding strength indicate that the production of formate on

metal surfaces proceeds through an oxygen bound interme-

diate *OCHO[50] as depicted in Scheme1a. ATR-IR studies on

the effect of surface oxidized species of the types MOx in Sn-

and In-based catalysts[41,43] indicate an alternative mechanism.

These studies conclude that the presence of an oxide layer on

Sn and In surfaces leads to the formation of hydroxylated sur-

faces (MOH) in aqueous media that react with dissolved CO2

to form an adsorbed carbonate (MCO3−) intermediate that is

proposed to undergo a two-electron reduction to yield formate

as depicted in Scheme1b. The mechanism proceeding through

an MCO3− was also found viable by DFT studies conducted

on hydroxyl terminated SnO surfaces.[51] The proposed mecha-

nisms implicate the persistence of SnOx or InOx surface spe-

cies at potentials relevant for CO2RR. A recent investigation on

SnO2 nanoparticles (4nm) supported on graphene by in situ

Raman spectroscopy and XAS[52] revealed that SnO2 and SnO

surface moieties persist at the catalyst surface in the potential

range where the best formate faradaic efficiency is observed

(–0.8V vs RHE in KHCO3 pH 8.5); providing further evidence

that persistent oxides play a key role in CO2RR.

To summarize, Indium has been consistently found to pro-

vide higher faradaic efficiencies at larger overpotentials than

Tin.[7,9,13,16,17,40–43] For both electrode materials, recent reports

point out that MOx enriched surfaces (with M being In or Sn) or

M0/MOx composites both boost the faradaic efficiency toward

formate. However, in view of the overall system conductivity, a

compromise must be met between the amount of metal and

metal oxide to preserve the material's high conductivity.[38,42]

In this context, amorphous bimetallic indium tin oxide thin

films (SnInOx)[53–56] appear as an interesting class of material

for CO2RR electrocatalysis. Such amorphous yet conductive

SnInOx thin films enable the investigation of possible syner-

gies between Sn and In for the electrocatalytic conversion of

CO2 to formate, while simultaneously providing an oxygen-rich

matrix for the formation of stable metal oxide surface species

(such as InOx and/or SnOx), known to favor the production of

formate with high faradaic efficiency. In the present paper, the

study of amorphous SnInOx thin films as (pre)electrocatalysts

for oxide-derived CO2RR reduction catalysts as well as a detailed

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investigation of their activity toward CO2 reduction along with

in-depth insights into material reduction and restructuration

upon CO2RR reaction are presented.

Our results revealed that SnInOx thin films act as pre-cat-

alysts for the in situ formation under CO2 electroreduction

conditions of metal@metal oxide core@shell nanoparticles,

composed of Sn-rich Sn1-XInX alloyed nanocores (with x<0.2)

surrounded by InOx-rich bimetallic In1-YSnYOZ shells (with 0.3

< y<0.4 and z≈ 1). The investigation of these in situ formed

nanoparticles under diverse applied electrolysis potentials and

extended electrolysis time revealed a dynamic particle size evo-

lution during CO2RR. Interestingly, despite the particle size

changes the “Sn1-XInX@In1-YSnYOZ” core@shell structures

with a ≈3nm oxide surface layer were found to be intrinsically

favored in all conditions essayed. Similar In surface segregation

in form of In(OH)3 has been previously observed under CO2RR

conditions for bimetallic Cu–In catalysts.[57] Such core@shell

structures offer potential advantages for electrocatalysis. For

instance, they maximize the exposition of the shell component

to the electrolyte as has been used to increase utilization of

precious metals for ORR application.[58,59] They can also offer

the possibility of tuning CO2R selectivity through cooperation

between core and shell components as has been demonstrated

for Ag@SnOx[60,61] and Cu@In(OH)3[57] catalysts for CO2RR.

2. Results and Discussion

Amorphous SnInOx thin films with a film thickness of

20–25 nm were deposited onto glassy carbon plate-like sub-

strates by spin coating using a single source precursor approach

(for details see experimental part as well as HR-SEM and TEM

images in Figure S1, Supporting Information). The composition

of the as-obtained SnInOx thin films was investigated by energy

dispersive x-ray spectroscopy (EDX), induced coupled plasma-

optical emission spectroscopy (ICP-OES), and x-ray photoelec-

tron spectroscopy (XPS), revealing an average In:Sn ratio of 0.4

(Table1). The CO2RR electrocatalytic activity of the SnInOx film

was evaluated in a two-compartment cell provided with a con-

stant inlet of CO2 at 30mLmin−1 using CO2 saturated 0.1m

KHCO3 (pH 6.8) as an electrolyte. A fritted tube was used to

separate the counter electrode to prevent re-oxidation of CO2RR

products. Further details can be found in the experimental

section and supporting information (Figure S2a, Supporting

Information). The electrolysis experiment comprised a linear

sweep voltammetry (LSV) in the reductive direction from

+0.4V to −1.0V versus RHE followed by a chronoamperometry

at −1.0Vversus RHE for 10 min. The gaseous products such

as H2 and CO were analyzed by gas chromatography, while the

products in the liquid phase were analyzed by 1H-NMR.

In the cathodic LSV in CO2 saturated electrolyte, the SnInOx

films exhibit an intrinsic reduction peak at −0.58V versus RHE,

which appears to overlap with the onset of CO2RR and corre-

sponds to the reduction of bimetallic SnInOx to the respective

metallic form (Figure1a and inset in Figure 1a). The overall

catalytic current reaches −7.8mAcm−2 at −1.0V versus RHE.

During the chronoamperometry step, the current decays within

the first minutes and stabilizes at −6.4mAcm−2 (Figure1b).

The product analysis revealed that the main CO2 reduction

product is formate with a faradaic efficiency of 80%. Addition-

ally, CO was produced with a faradaic efficiency of 5.4%, while

H2 was produced with a faradaic efficiency of 14.5% (Figure1c).

For comparison, a cathodic LSV was performed for an

SnInOx film in CO2-free, N2-saturated electrolyte. Under

these conditions, only HER is expected. During the LSV the

SnInOx intrinsic reduction peak appears at −0.54 V versus

RHE (Figure1a). The catalytic current sets on at lower poten-

tial (−0.65V vs RHE) than it was observed in the presence of

CO2 and reaches −5.2mAcm−2 at −1.0V versus RHE, while

decaying to −3.7 mAcm−2 during the 10min chronoamperom-

etry test. The product analysis revealed, as expected, the sole

production of H2 with 100% faradaic efficiency (Figure1c).

The electrolysis experiments conducted in both N2 and CO2

saturated electrolytes (Figure1a) revealed that both CO2 reduc-

tion and H2 evolution onset after the electrochemical reduction

of the SnInOx films. The extent of SnInOx film reduction will

be discussed in detail in the next section (vide infra). In short,

TEM and SEM characterization revealed that the amorphous

SnInOx thin films act as precatalysts and are reduced in situ

during the CO2RR reaction to form core@shell nanoparticles

composed of Sn-rich metallic alloy cores of the type Sn1-XInX

(x<0.2) surrounded by indium-rich indium tin oxide shells of

the type In1-YSnYOZ with y= 0.4 and z≈ 1, denoted “Sn1-XInX@

In1-YSnYOZ”core@shell in the following.

It is important to note that HER activity is heavily sup-

pressed in CO2 saturated electrolyte for these SnInOx-derived

Scheme 1. Mechanisms of formate production for In- or Sn-based catalysts proposed in literature: a) Production of formate via *OCHO interme-

diate,[50] b) Production of formate via M-CO3− intermediate.[41,43] Note: M corresponds to Sn or In and MOx refers to surface oxide species on these

metal surfaces.

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nanoparticle films (Figure1b,c). Indeed, in situ reduced SnInOx

provides a partial HER catalytic current density of −3.7mAcm−2

in N2 saturated electrolyte and a 100% faradaic efficiency toward

H2 evolution. However, in CO2 saturated electrolyte, the fara-

daic efficiency for H2 evolution drops down to 14.5% accounting

for a H2 partial current of only −0.92mAcm−2. These results

indicate that although the in situ reduced SnInOx does cata-

lyze H2 evolution, in the presence of CO2 it exhibits a marked

affinity and higher activity toward the activation of CO2.

The affinity of the SnInOx derived composites for CO2 can fur-

ther be observed by comparing the reduction of SnInOx in N2 and

CO2 saturated electrolyte by voltammetry (Figure1a,d). The inset

in Figure1a displays the redox peaks observed upon the reduction

of SnInOx films in the first cathodic sweep in N2 and CO2 satu-

rated electrolyte, respectively. In N2-saturated electrolyte, a sharp

reduction peak ascribed to the reduction of InOx (Inδ+) to metallic

In0 and SnOx (Snδ+) to metallic Sn0 is observed at −0.54V, while

the onset of hydrogen evolution current is observed at higher

reductive bias. In the presence of CO2 however, the redox peak

located at −0.58V is less sharp and overlaps with the onset of CO2

reduction current. It is during this first reduction sweep that the

SnInOx film is transformed into “Sn1-XInX@In1-YSnYOZ”core@

shell nanoparticles. Figure1d displays a stable CV profile in N2

and CO2 saturated electrolyte after 5 cycles at 100 mVs−1. While

the redox peaks associated to the InSnOx reduction are still clearly

seen in N2 saturated electrolyte, their intensity is strongly dimin-

ished in the presence of CO2. This behavior has been previously

observed for both Sn[43] and In[41,42] based catalysts and has been

ascribed to the reaction of SnOx and InOx surfaces moieties with

dissolved CO2 to form a surface confined M-CO3− intermediate

(Scheme1b). This intermediate is reduced during CO2RR cata-

lytic turnover (i.e., at larger reductive bias) to yield formate and

the metal site in its oxidized state, thus decreasing the magnitude

of the MOx/M0 redox peaks.

As already mentioned, “Sn1-XInX@In1-YSnYOZ” core@shell

nanoparticles derived from in situ reduction of SnInOx provide

a high CO2RR catalytic activity with a formate faradic efficiency

of 80% at an overall current density of −6.4± 0.5mAcm−2 at

−1.0V versus RHE, corresponding to a formate production rate

of 95.5 µmol h−1 cm−2. Considering the ultra-low mass of In

and Sn present in the SnInOx films as determined by ICP-OES

(m(Sn+In)= 12µgcm−2, mSn= 8 µgcm−2, mIn= 4.3µgcm−2),

this performance corresponds to a remarkably high formate

mass activity of 437± 15Ag−1(Sn+In) when referred to the total

mass of Sn and In, as well as to a (theoretically) high mass

activity of 640± 21AgSn−1 and 1190± 50AgIn−1 when referred

to the mass of Sn and In separately, assuming each metal as

the single CO2RR active component. With this mass activity,

the SnInOx-films derived electrocatalysts presented in this work

outperform the best reported Sn and closely match the best

reported In based electrocatalysts reported in H-cell configura-

tion so far. For comparison, the best Sn nanoparticulate system,

that is, in situ reduced SnO2 nanoparticles (diameter = 5nm)

supported on graphene,[40] was reported to have a mass activity

of 266AgSn−1 at −1.16V versus RHE, while the most active In

based catalysts, that is, In nanoparticles (diameter = 6.1 nm)

and In2O3 nanoparticles (9.77nm) supported on Vulcan were

reported with 215 AgIn−1 and 450 AgIn−1at −0.96V versus RHE

respectively.[42] Further reports and comparisons of activity and

selectivity of In and Sn-based electrocatalysts can be found in

Table S1, Supporting Information.

2.1. In Situ SnInOx Film Reduction and Restructuring during

CO2RR Electrolysis

As pointed out previously, the SnInOx films act as precatalysts

and are reduced in situ during CO2 electrolysis and strong

Table 1. Composition of as synthesized SnInOx films and in situ formed “Sn1-XInX@In1-YSnYOZ” core@shell nanoparticles during CO2 reduction

determined by different analytical methods (ICP, EDX, XPS).

Technique Sna) Ina) O/Mb) In:Sn

As synthesized

SnInOx films

ICP 0.69 0.31 – 0.44

SEM-EDXc) 0.71 0.29 1.5 0.40

XPS (500eV)c) 0.78 0.22 2.0 0.29

Sn1-XInX@In1-YSnYOZ”

core@shell

particles

SnInOx_CAT@-1V_10min

ICP (bulk) 0.63 0.37 – 0.58

SEM-EDX (bulk)c) 0.63 0.37 3.3-6.3 0.59

STEMc) Core 0.85 0.15 0.4 0.18

Shell 0.14 0.86 1.9 6.05

XPS*d) 200 eV 0.42 0.58 – 1.33

500 eV 0.45 0.55 1.1 1.23

1000 eV 0.48 0.52 – 1.10

XPSc) 500 eV 0.42 0.58 1.7 1.39

a)Sn and In fraction referred to the total metal content; Sn = at%Sn/(at%Sn+ at%In); b)Oxygen to total metal ratio O/M = at%O/(at%Sn+ at%In); c)Characterization con-

ducted on samples exposed to air; d)* denotes characterization conducted on samples handled, stored, and transferred under inert atmosphere.

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2103601 (5 of 14)

structural changes occur. In order to investigate these changes,

the SnInOx films were subjected to different CO2 electrolysis

experiments and the resulting samples were denoted SnInOx-

CAT@E-t, where E is the electrolysis potential (vs RHE) and t

is the duration of the CO2 electrolysis test (i.e., the duration of

the chronoamperometry step in the presence of CO2). The post

electrolysis samples SnInOx-CAT@E-t were then characterized

by ex situ by SEM, TEM, STEM, and GI-RXD (Note that unless

otherwise specified, the ex situ characterization of samples is

conducted on samples exposed to air after electrolysis tests).

After a typical CO2RR electrolysis experiment at −1.0V versus

RHE held for 10 min, denoted SnInOx_CAT@-1V_10min, the

originally planar and continuous SnInOx thin film (see SEM

top view image, Figure2b) is restructured into an array of nan-

oparticles with a size mode between 30 and 35nm (SEM top

view image, Figure2c). The overall metal composition of the

SnInOx_CAT@-1V_10minsamples was assessed and EDX (per-

formed in SEM top view mode, Figure S8, Supporting Informa-

tion) and ICP-OES analysis revealed an overall In:Sn ratio of 0.6

(Table1). Compared to the initial In:Sn ratio of 0.4 found for the

starting SnInOx films, the composition of the reduced samples

indicates a partial dissolution of Sn (≈1.5–2µgcm−2) during the

in situ reduction of planar SnInOx films to nanoparticles.

A detailed examination of the formed nanoparticles in a

sample after 10min of CO2 electrolysis at −1Vversus RHE (i.e.,

in SnInOx_CAT@-1V_10min)by transmission electron micros-

copy (TEM) (Figure2d,f) revealed that each particle possesses a

core@shell structure. From high-resolution transmission elec-

tron microscopy (HR-TEM) images (Figure2d), a single crystal-

line core with a d-spacing of 2.8Å (Figure2d) was identified;

Figure 1. Electrocatalytic tests in aqueous 0.1m KHCO3 performed with SnInOx planar films deposited on Glassy Carbon plate electrodes (GCE):

a) cathodic linear sweep voltammetry (first sweep at 5mVs−1, inset displays an enlarged figure of the redox peak region associated to SnInOx reduc-

tion), followed by b) chronoamperometry at −1.0V versus RHE for SnInOx in CO2 saturated electrolyte (SnInOx_CO2, dark green) and SnInOx in N2

saturated electrolyte (SnInOx_N2, orange). Note: The blank measurements were done with pristine GCE plates. c) Faradaic efficiencies toward products

such as formate (green bar), carbon monoxide (red bar) and H2 (grey bar) and respective current density (black dot). d) Stable cyclic voltammetry

profile (typically after the fifth cycle) of SnInOx films cycled at 100mVs−1 in CO2 (dark green) or N2 (orange) saturated 0.1 m KHCO3 and inset showing

the potential region where SnInOx redox peaks occur.

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