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2D Metal Oxyhalide Derived Catalysts for Efficient CO2 Electroreduction

F. Pelayo García de Arquer, Oleksandr S. Bushuyev, Phil De Luna, Cao-Thang Dinh, Ali Seifitokaldani, Makhsud I. Saidaminov, Chih-Shan Tan, Li Na Quan, Andrew Proppe, Md. Golam Kibria, Shana

Kelley, David Sinton, and Edward H. Sargent

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Citation

(published version)

García de Arquer, F. Pelayo, Oleksandr S. Bushuyev, Phil De Luna,

Cao‐Thang Dinh, Ali Seifitokaldani, Makhsud I. Saidaminov, Chih‐Shan

Tan et al. "2D Metal Oxyhalide‐Derived Catalysts for Efficient CO2

Electroreduction." Advanced Materials 30, no. 38 (2018). Doi: 10.1002/adma.201802858

Publisher’s Statement This is the peer reviewed version of the following article: García de Arquer, F. Pelayo, Oleksandr S. Bushuyev, Phil De Luna,

Cao‐Thang Dinh, Ali Seifitokaldani, Makhsud I. Saidaminov, Chih‐Shan

Tan et al. "2D Metal Oxyhalide‐Derived Catalysts for Efficient CO2

Electroreduction." Advanced Materials 30, no. 38 (2018) which has been published in final form at This article may be

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DOI: 10.1002/((please add manuscript number))

Article type: Communication

2D Metal Oxyhalide Derived Catalysts for Efficient CO2 Electroreduction

F. Pelayo García de Arquer†,1,2, Oleksandr S. Bushuyev†,1,3, Phil De Luna†,4, Cao-Thang

Dinh1, Ali Seifitokaldani1, Makhsud I. Saidaminov1, Chih-Shan Tan1, Li Na Quan1, Andrew

Proppe1, Md. Golam Kibria1, Shana Kelley3, David Sinton2, Edward H. Sargent1*

Dr. F. P. G.d.A., Dr. O. S. B., Dr. C.-T. D., Dr. A. S., Dr. M. I. S., Dr. C-S. T, Dr. L. N. Q., A.

P, Dr. M. G. K., Prof. E. H. S.

Department of Electrical and Computer Engineering, University of Toronto, 35 St George

Street, Toronto, Ontario M5S 1A4, Canada

E-mail: ted.sargent@utoronto.ca

P. D. L,

Department of Materials Science Engineering, University of Toronto, 27 King's College

Circle, Toronto, Ontario M5S 1A1, Canada

Dr. O. S. B., Prof. S. K,

Leslie Dan Faculty of Pharmacy, Faculty of Medicine, Biochemistry, University of Toronto,

Toronto, ON M5S 3M2, Canada

Dr. F. P. G.d.A., Prof. D. S.

Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s

College Rd., Toronto, Ontario M5S 3G8, Canada

Keywords: ((maximum five, not capitalized, plural, separated by commas, no full stop))

† these authors contributed equally to this work

2

Electrochemical reduction of CO2 is a compelling route to store renewable electricity in

the form of carbon-based fuels. Efficient electrochemical reduction of CO2 requires

catalysts that combine high activity, high selectivity, and low overpotential. Extensive

surface reconstruction of metal catalysts under high productivity operating conditions

(high current densities, reducing potentials, and variable pH) renders all the more

challenging the realization of tailored catalysts that maximize the exposure of the most

favourable facets, the number of active sites, and the oxidation state. Earth-abundant

transition metals such as tin, bismuth, and lead have been proven stable and product-

specific, but exhibit limited partial current densities. Here we report a new strategy that

employs bismuth oxyhalides as a template from which we derive 2D bismuth-based

catalysts. The BiOBr-templated catalyst exhibits a preferential exposure of highly active

Bi(11 0) facets. We increase thereby CO2 reduction reaction selectivity to over 90%

Faradaic Efficiency; and simultaneously achieve stable current densities up to 200

mA·cm-2 - more than a 2-fold increase in the production of the energy-storage liquid

formic acid compared to previous best Bi catalysts.

Electrochemical reduction of CO2 to fuels and feedstocks, powered using renewable

sources of electricity, offers to contribute to closing the carbon cycle.[1–5] Specificity and

productivity are measured via Faradaic efficiency (FE) and current density. Until now, the most

productive electrocatalysts for formate – an attractive candidate as a liquid fuel and feedstock

–[6–9] are derived from noble metals such as palladium and silver.[10–13]

Recent research efforts have focused on finding non-noble earth-abundant catalysts for

formate production. Tin, bismuth and lead have shown high stability and selectivity.[10,14–25]

Despite the vast choice of catalysts, their productivity has generally not been sufficiently

high.[22] They have suboptimal binding energies for CO2 reduction intermediates and suffer

3

from by-product poisoning at certain facets. The extensive surface reconstruction of metal

catalysts under high current operating conditions makes it particularly challenging to program

specific favourable facets, active sites, and oxidation states, during operation.[26–28]

Here we report bismuth metal catalysts prepared via the in-situ restructuring of 2-

dimensional bismuth oxyhalides. The new catalysts exhibit enhanced specificity and

productivity relative to previously-reported bismuth catalysts.[29] They reach current densities

above 200 mA·cm-2 at over 90% FE in the electroproduction of formate.

We achieved this advance by taking advantage of 2-dimensional layered precursor

materials, bismuth oxyhalides. The favourable formation energy of this pre-catalyst leads to

selective reconstruction upon in situ electroreduction to form high-surface-area petal-structured

electrodes. This results in the preferential exposure of more active Bi (11 0) facets, which we

demonstrate with in-situ grazing-incidence wide-angle X-ray scattering (GIWAXS) and X-ray

absorption spectroscopy (XAS). This strategy increases the CO2 reduction reaction specificity

to over 90% while maintaining record-high current density (Table 1).

Results and discussion

In light of recent work reporting in-situ formation of optimal CO2RR catalysts via

operando electro-redeposition,[26,27] we explored bismuth-based compounds as a materials

platform with which we could deliberately control the morphology to promote efficient

catalysis. The bismuth oxide sheets making up oxyhalides (BiOX), where X = I, Br, Cl, are

separated by negatively charged halogens (Figure 1a).[30] We hypothesized that these materials

would, upon electroreduction, generate highly active ultrathin layers of metallic bismuth as the

halogen layer is dynamically removed.[29,31,32]

We prepared catalyst samples by coating the BiOBr in dimethyl sulfoxide (DMSO)

solution onto carbon paper electrodes and annealing in an inert atmosphere (see methods). The

crystal structure of the resulting precatalyst was confirmed using X-ray diffraction, and matched

well with the calculated peaks for this structure (Figure 1b). The precursor catalysts were then

4

electroreduced in CO2 saturated aqueous KHCO3 solutions via 10 cycles of cyclic

voltammograms (CV). After this electroreduction preconditioning, the sample consisted of a

combination of Bi and minor Bi2O3 species, which was then used as a catalyst without further

modification. X-ray Photoemission Spectroscopy (XPS) reveals the stoichiometry

reconfiguration after sample hydration toward a Br-rich BiOBr, and the presence of residual Br

in the reduced samples (Suppl. Fig. S1). High-Resolution Transmission Electron Micrograph

(HR-TEM) images reveal the presence of various BiOBr facets that selectively reconfigure into

Bi (110) following conditioning (Figure 1c-d). Scanning Electron Micrograph (SEM) images

of the BiOBr-templated catalysts before and after reconfiguration show a transition from an

isotropic and amorphous configuration into a 2-dimensional ultrathin petal-like arrangement

(Figure 1e-f).

To elucidate the structural evolution of the BiOX-templated catalyst at the atomic level,

we carried out in-operando GIWAXS measurements (Figure 2). BiOBr samples were placed

in a custom-made reactor compatible with GIWAXS measurements and studied at different

conditions (Figure 2a-b). At open-circuit conditions, and in the absence of electrolyte, scattering

peaks associated are seen in GIWAXS that we associate with BiOBr, Bi2O3 and Bi species

(Figure 2c). After we add 1M KHCO3 electrolyte, the signal to noise ratio decreases and only

the peak associated with BiOBr remains.

We then operated the catalyst under increasingly reducing conditions. At -1.5 V vs.

RHE, there is a significant catalyst reconfiguration; Bi (110) facets become the dominant

arrangement, with a suppression of bismuth oxide species and a decreased contribution of

BiOBr. This is in agreement with the Pourbaix phase diagram of BiOBr at this operating

condition (Suppl. Fig. S2), which predicts a configuration consisting of Bi (s) and Br- ions to

be the most thermodynamically favourable.

To shed light on the electronic structure of the BiOBr-templated catalyst at operating

conditions, we performed X-ray Absorption Near Edge Structure (XANES) and Extended X-

5

ray Absorption Fine Structure definition (EXAFS) on BiOBr and Bi samples (Figure 2d). The

absorption onset of the Bi L3 transition shifts toward higher energy for BiOBr samples, which

indicates an increasing oxidation state of as-prepared BiOBr compared to control Bi. BiOBr-

templated samples exhibit a reduction in the oxidation state after operation consistent with

BiOBr electroreduction. We resolved the Fourier transformed |χ(R)| spectrum to evaluate the

density of neighbouring atoms as a function of radial distance (Figure 2e).[33] A different

distribution of Bi-O, Bi-Br and Bi-Bi bonds is evident for Bi, BiOBr, and BiOBr-templated

samples. Both oxybromide and oxybromide-derived samples exhibit a modified presence of

closer-distance oxide neighbours and an apparent Bi-Bi shortening. BiOBr samples possess

their strongest peak at R = 1.8 A-1, which we attribute to the presence of O and Br neighbours.

BiOBr-templated samples show a reduction in the density of these states, in agreement with

XPS measurements, and instead exhibit two local maxima near 2.3 A-1 and 3.0 A-1. This is

consistent with a Bi (110) configuration with an azimuthal compression, in contrast to bulk Bi

control samples. We hypothesize that this favours the exposure of more active Bi facets as

observed by in situ GIWAX.

We then sought to evaluate the performance of the BiOBr-templated catalysts for CO2

electroreduction. Testing of these catalysts in an aqueous H-cell set-up (0.1 M KHCO3

electrolyte) revealed their higher activity compared to BiOx controls, exhibiting more than a

two-fold increase in current density which led to 80 mA·cm-2 at -1.0 V vs. RHE (Figure 3a).

This approaches the CO2 reduction current density limit achievable in the H-cell owing to CO2

mass transport limitations.[34]

To study the activity of the catalyst with respect to operating potential, we analysed the

selectivity of BiOBr catalysts toward formate at different reducing potentials (Figure 3b). At

potentials more negative than -1 V, hydrogen evolution accounted for less than 3% of product

and CO production was totally suppressed. The maximum HCOO- : H2 ratio was observed in

the range of -0.8 to -1.0 V vs. RHE after iR correction, with the formate Faradaic efficiency

6

reaching values over 99%. Liquid product analysis was performed using Nuclear Magnetic

Resonance (NMR) in water suppression mode (see methods). Bi control samples, on the other

hand, exhibited selectivities to formate of about 70% in this potential range. BiOBr-templated

catalysts exhibited remarkable stability, and retained their original performance during the

course of an initial 65 h study at continuous operation (Figure 3c). This represents a two-fold

improvement in current density at near-unity Faradaic efficiency compared to the best bismuth-

based catalyst for CO2 electroreduction into formate in traditional H-cell configurations. We

also sought to switch the selectivity of BiBrO-templated catalysts toward CO and, when we

operated it in an aqueous-free electrolyte, we achieved a similar productivity for this product

(Suppl. Fig. S3)

In view of the high current densities exhibited by BiOBr-templated catalyst, which

reached CO2 mass-transport limitations in an H-cell configuration, we developed catalysts that

would work in a flow-cell system in which gas and liquid media are separated,[4] overcoming

mass transport in aqueous electrolytes and thus allowing a much higher CO2RR current density

and thus overall system productivity. To this end, we deposited BiOBr on top of a gas diffusion

layer (GDL) carbon electrode as a seed for the final BiOBr-templated catalyst. This allowed us

to operate at much higher current densities in excess of 200 mA·cm-2 in a 2M KHCO3

electrolyte (Figure 3d and Suppl. Fig S4). The high selectivity toward formate remained largely

unaffected, reaching a record value of 90±5% even at these high current densities. Bi control

samples consistently exhibited lower current densities and formate selectivity around 70±5%.

To provide further mechanistic insights into formate production on the BiOBr-

templated catalyst, we performed density functional theory (DFT) calculations (Figure 4).[35]

Based on the operando GIWAXS experiments we focused on the Bi (121) and Bi (110) facets

to determine the effect of restructuring on the catalytic activity. From GIWAXS experiments,

we found the Bi (121) facet present in the precatalyst phase while Bi (110) emerged as the

dominant facet during operation. Only by measuring the surface facet structure under reaction

7

conditions were we able to develop model metal slab models representative of the experimental

catalyst. We sampled symmetric binding sites on the surface for both the HER (H*) and CO2RR

to formate (HCOO*) intermediates (Suppl. Fig. S5-S6) and chose the lowest energy structures

for further analyses.

We explored the reaction energy pathways for CO2 reduction to formate and the

competing hydrogen evolution reaction (HER). Due to the near unity experimental selectivity

for formate, we did not focus on the CO production pathway. We found that the reaction energy

barrier (ΔG) was significantly higher for both HER (0.90 eV) and CO2RR (0.74 eV) on the Bi

(121) facet. However, the Bi (110) facet displayed significantly lower reaction free energies

for both HER (0.14 eV) and CO2RR (0.03 eV). Importantly, the reaction energy barrier for

CO2RR to formate is the lowest of all reactions on the Bi (110) facet with the rate determining

step being the second proton-coupled electron transfer to the bound HCOO* intermediate.

Additionally, the stepped Bi (110) surface offers near optimal Gibbs free binding energies for

HCOO* (-0.01 eV) close to the thermodynamic minimum. The GIWAX, electrocatalytic

experiments, and DFT studies, taken together, point towards the promotion of a highly active

surface facet during CO2 reduction that favours formic acid production to the exclusion of

competing products.

Conclusions

We demonstrate a new catalyst design strategy that by beginning with bismuth oxyhalides as a

template from which to derive 2D bismuth-based materials enables the realization of catalysts

that can sustain simultaneously high selectivity and activity for CO2 electroreduction. We

employed in-situ GIWAX measurements to demonstrate that BiOBr-templated catalysts exhibit

a preferential exposure of more highly active Bi (110) facets. We achieve thereby a high

selectivity toward formate over 90% that remains up to current densities as high as 200 mA·cm -

2, demonstrating the potential of metal oxyhalide templated catalysts for the efficient

electroreduction of CO2.

8

Experimental Section

An extensive description of materials, sample fabrication and characterization, product analysis

and computational studies, can be found the supplementary information.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

This work was supported by the Canadian Institute for Advanced Research (CIFAR) Bio-

inspired Energy Program, the Ontario Research Fund (ORF-RE-08-034), and the Natural

Sciences and Engineering Research Council (NSERC) of Canada. The authors thank the

Canadian Light Source (CLS) for support in the form of a travel grant. Computations were

performed on the SOSCIP Blue Gene/Q computing platform. SOSCIP is funded by the Federal

Economic Development Agency of Southern Ontario, the Province of Ontario, IBM Canada

Ltd., Ontario Centres of Excellence, Mitacs and 15 Ontario academic member institutions. The

authors thank X. Gong, J. Li, R. Quintero-Bermudez, P. Cheng, J. Li, L. Levina, R. Wolowiec,

D. Kopilovic, and E. Palmiano for their help over the course of this research. M.I.S.

acknowledges the support of Banting Postdoctoral Fellowship Program, administered by the

Government of Canada. O.S.B. and P.D.L. acknowledges the financial support in the form of

the NSERC Post-Doctoral Fellowship and Canada Graduate Scholarship – Doctoral (CGS-D)

Award, respectively.

Received: ((will be filled in by the editorial staff))

Revised: ((will be filled in by the editorial staff))

Published online: ((will be filled in by the editorial staff))

9

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13

Figure 1 | Bismuth oxyhalide templated catalysts. (a) Atomic structure of BiOBr and BiOBr-

templated Bi catalyst after electroreduction. (b) X-Ray diffraction patterns of BiOBr precatalyst

and BiOBr-templated samples, which are obtained after hydration of BiBr/carbon paper. The

peak at 26° is that of the carbon support. BiOBr-templated catalysts are obtained by

electroreducing BiOBr in a CO2-purged electrolyte. The crystal structure of BiOBr-templated

catalysts consists of a combination of Bi and Bi2O3 species. (c-d) High-resolution Transmision

Electron Micrographs (HRTEM) before and after catalyst conditioning revealing the presence

of different BiOBr facets (hydrated, before reaction) and their selective reconstruction into Bi

after operation (e-f) Scanning Electron Micrographs of samples after reaction reveal a 2-

dimensional petal-like layered arrangement after BiOBr electroreduction.

14

Figure 2 | Surface reconstruction and in-situ characterization of BiOBr-templated

catalysts. (a) In-situ grazing-incidence wide-angle X-ray scattering (GIWAXS) of BiOBr

samples at open circuit potential and (b) under reducing potential vs. RHE showcasing the

catalyst reconfiguration. (c) GIWAXS intensity along qy = 0 revealing the drastic reduction of

oxide and oxyhalide species and the dominant presence of Bi (110) of Bi (121) facets. This is

supported by HRTEM measurements (Fig. 1c,d). Near qy = 1.6 A-1 various BiOBr and Bi-oxide

contributions overlap. (d) X-ray absorption near edge structure (XANES) of Bi L3 transition

revealing the different oxidation state of metal Bi and BiOBr derived samples. (e) Extended X-

15

Ray Absorption Fine Structure (EXAFS) of Bi, BiOBr and BiOBr-templated samples showing

the increasing contribution of Bi-O bonds and compression of interplane Bi-Bi distances in

BiOBr-derived samples.

16

Figure 3 | BiOBr-templated catalysts for efficient and stable CO2 electroreduction. (a)

Linear-sweep- voltammograms of BiOBr-templated catalysts vs. thermally evaporated Bi

control at 1M KHCO3 after iR-correction. BiOBr samples exhibit a lower onset potential and a

significantly higher current density; (b) product distribution for different operating potentials.

H2 production is suppressed after -0.75 V vs. RHE, where only formate can be detected. At

currents approaching 80 mA·cm-2 there is a rise in H2 production due to CO2 mass-transport

limitations. Reference Bi samples, on the other hand, exhibit consistently a peak FE for formate

lower than 75%; (c) current density trace at a -0.9 V vs. RHE potential. The Faradaic efficiency

for H2 throughout the run is less than 4% throughout, and the cumulative FE for formate

approaches unity. (d) Faradaic efficiency as a function of current in H-cell and flow-cell

17

configurations. A FE of 90% is sustained up to 200 mA·cm-2. The performance of other non-

noble metal catalyst for CO2 electroreduction is shown in Table 1 for comparison.

18

Figure 4 | Reaction energy diagrams for HER (green) and CO2RR to formic acid (blue)

on (a) Bi (121) and (d) Bi (110) facets. Schematic of H* and HCOO* adsorption sites on

(d,e) Bi (110) and (c,f) Bi (121) . Bi (110) exhibits a preferential energy landscape for

CO2RR.

19

Catalyst Current Density

(mA/cm2) Formate F.E.

(%) Citation

BiBrO-templated 200 90+ This work

Metallic Bi 80 70 This work

Bi/ionic liquids 40 80 ACS Catalysis 8, 2857

(2018)[19]

Bi/ionic liquids 12.5 90+ ACS Catalysis 8, 931

(2018)[36]

SnS-derived 55 90+ Joule 1, 794–805

(2017)[10]

SnO nanoparticles 200 64 J. Mater. Chem. A, 6, 10313

(2018)[37]

Sn 200 90 J. Appl. Electrochem., 44,

1107, (2014)[15]

CoOx 10 90 Nature 529, 68 (2016)[32]

Pb 62 95 Renewable Energy 95, 277,

(2016)

Ag 300 60 J. Am. Chem. Soc., 140,

3833 (2018)[38]

Table 1 – Performance comparison of various non-noble metals for CO2 electroreduction to

formate and best non-noble metal.

20

2D Bismuth oxyhalide templated catalyst for CO2 electroreduction exhibit an in

operando preferential facet exposure which enables sustaining near unity selectivity to

formate production even at high current densities up to 200 mA·cm-2.

Keyword: CO2; electroreduction; formate; catalysis; 2D materials

F. P. G.d.A., O. S. B., P. D. L., C.-T. D., A. S., M. I. S., C.-S. T., L. N. Q., A. P, M. G. K., S.

K., D. S., E. H. S.

2D Metal Oxyhalide Derived Catalysts for Efficient CO2 Electroreduction

ToC figure

21

Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2016.

Supporting Information

Supplementary Information for:

2D Metal Oxyhalide Derived Catalysts for Efficient CO2 Electroreduction

F. Pelayo García de Arquer†,1,2, Oleksandr S. Bushuyev†,1,3, Phil De Luna†,4, Cao-Thang

Dinh1, Ali Seifitokaldani1, Makhsud I. Saidaminov1, Chih-Shan Tan1, Li Na Quan1, Andrew

Proppe1, Md. Golam Kibria1, Shana Kelley3, David Sinton2, Edward H. Sargent1*

1Department of Electrical and Computer Engineering, University of Toronto, 35 St George

Street, Toronto, Ontario M5S 1A4, Canada

2Leslie Dan Faculty of Pharmacy, Faculty of Medicine, Biochemistry, University of Toronto,

Toronto, ON M5S 3M2, Canada

3Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s

College Rd., Toronto, Ontario M5S 3G8, Canada

4Department of Materials Science Engineering, University of Toronto, 27 King's College

Circle, Toronto, Ontario M5S 1A1, Canada

† these authors contributed equally to this work

* corresponding author: ted.sargent@utoronto.ca

Index:

S1. X-Ray Photoemission Spectroscopy Measurements

S2. Pourbaix diagram of BiBrO in aqueous media

S3. Switching BiBrO selectivity

S4. Methods

- Density Functional Theory

- Sample fabrication

- Material characterization

- Electrochemical characterization

- Product characterization

22

Supplementary Section S1

| X-Ray Photoemission Spectroscopy Measurements

Figure S1. X-Ray Photoemission Electron Spectroscopy

23

Supplementary Section S2

| Pourbaix diagram of BiOBr in aqueous media

Figure S2. Pourbaix diagram of BiOBr in aqueous media as calculated from

materialsproject.org

24

Supplementary Section S3

| Switching BiOBr selectivity

Bi-based catalysts were previously optimized for CO evolution when utilized in non-aqueous

media, a finding that prompted us to evaluate the BiOBr-templated catalyst for CO production.

For this experiment we replaced the electrolyte with 1 M tetraethyl ammonium bromide

(TEAB) in acetonitrile (ACN) while utilizing an Ag wire as a reference electrode. Catalyst

activity in the TEAB/ACN mixture was very similar to that observed in the aqueous electrolyte,

reaching 60 mA/cm2 at potentials below - 1.4 V vs RHE, but with CO as a main reaction product

(Figure S3). We observed as high as 67% of current going towards production of CO,

corresponding to a partial current density above 40 mA·cm-2. Up to a 20-25% of current was

driven towards formate production, likely because of concurrent water evolution reaction

switching the reaction pathway towards formate generation. These findings represent a 70%

increase in CO partial current density compared to best Bi catalysts operated toward CO

electroproduction.

Figure S3. (a) CV scan of a BiOBr-templated sample on aqueous-free electrolyte in an H-cell.

(b) The Faradaic efficiency for different potentials vs. RHE (not iR corrected) shows a CO

selectivity near 70%.

25

Supplementary Section S4

| Flow-cell operation

Figure S4. (a) LSC scan of a BiOBr-templated sample on 2 M KCHO3 electrolyte in a flow

cell configuration (iR corrected).

26

Supplementary Section S5: Methods

Density Functional Theory

The Vienna Ab Initio Simulation package (VASP)[39] was used to perform all density

functional theory (DFT) calculations. The revised Perdew-Burke-Ernzerhof (RPBE)[40]

generalized gradient approximation (GGA) exchange correlation functional was used as it has

been shown to accurately calculates the chemisorption energetics of atoms on p-block metal

surfaces. All-electron frozen-core projector augmented wave[41] (PAW) pseudopotentials with

a Bloch[42] plane wave basis set were used with a cutoff energy of 500 eV with a Fermi smearing

width of 0.1 eV and dipole corrections included. Monkhorst-Pack mesh[43] was used for k-point

sampling with 2x2x1 k-points sampled for the Bi(1,2,1) slab and 4x3x1 k-points samples for

the Bi(1,-1,0) slab. The 48 atom slabs were constructed using the open-source atomic simulation

environment (ASE) code[44]. The slabs with adsorbed intermediates were modelled with a

vacuum space of at least 10 Å perpendicular to the surface. Spin polarization was included as

it has previously shown to be important for binding energies on metal and metal oxide catalyst

surfaces.[45] Structural and unit cell optimizations were performed with the Broyden-Fletcher-

Goldfarb-Shanno (BFGS) algorithm until the maximum structural optimization cutoff was

reached (0.02 eV/atom). After slab optimization, all subsequent thermodynamic calculations

were performed with the bottom two layers fixed to simulate interactions with the bulk.

The Gibbs free energies were calculated at 298K and 1 atm as outlined below:

𝐺 = 𝐻 − 𝑇𝑆 = 𝐸DFT + 𝐸ZPE + ∫ 𝐶vd𝑇298

0

− 𝑇𝑆

where 𝐸DFT is the DFT calculated electronic energy, 𝐸ZPE is the zero-point vibrational energy,

∫ 𝐶vd𝑇298

0 is the heat capacity, T is the temperature, and 𝑆 is the entropy. Gas phase molecules

such as CO2 and H2 were treated using the ideal gas approximation while adsorbates were

27

treated using a harmonic approximation. The DFT calculated energy for gas phase CO2,

HCOOH, H2 were corrected by 0.41 eV, 0.20 eV, and 0.09 eV, respectively, to account for

systematic errors by DFT.[35] The computational hydrogen electrode model (CHE)[46] was used

to calculate the change in Gibbs free energy, ΔG, between reaction steps of the CO2 to formate.

The reactions steps for the electrochemical reduction of CO2 to formate and the

hydrogen evolution reaction are as follows:

Formic Acid:

CO2 + H+(aq) + e- + * → HCOO* (1)

HCOO* + H+(aq) + e- → HCOOH (2)

Hydrogen:

H+ + e- + * → H* (5)

H* + H+ + e- → H2 (6)

where * represents either a vacant surface catalytic active site, or intermediate species adsorbed

on the active site.

Since Bi(1,-1,0) and Bi(1,2,1) are not typical facets that have been modelled

computationally, we performed a screen on multiple symmetry equivalent adsorption sites on

the stepped and undercoordinated surfaces of the (1,-1, 0) and (1,2,1) facets (Figure S1).

28

Figure S5. Different adsorbate binding positions sampled for H* on (a) Bi(1,-1,0) and (b)

Bi(1,21) and HCOO* on (c) Bi(1,-1,0) and (d) Bi(1,2,1).

We pick the lowest Gibbs free energy of adsorption as representative of the most likely

mechanism for CO2RR and HER. The Gibbs free energy of adsorption is defined as below:

𝛥𝐺𝑎𝑑𝑠 = 𝐺𝑎𝑑𝑠𝑜𝑟𝑏𝑎𝑡𝑒∗ − (𝐸𝑠𝑙𝑎𝑏 + 𝐺𝑔𝑎𝑠)

where Gadsorbate* is the Gibbs free energy of the adsorbed intermediate species (HCOO* or H*)

on the slab, Eslab is the electronic energy of the slab (entropic contributions are neglected for the

slab), and Ggas is the reference intermediate where Ggas = ½ GH2 for HER and Ggas = GHCOOH –

½ GH2. Interestingly, it was found that the Bi(1,2,1) facet had fairly consistent adsorption

energies regardless of binding position with only a 0.48 eV difference between the highest and

lowest energies for H*. In contrast, the Bi(1,-1,0) facet exhibited a larger spread in H* binding

(0.70 eV), most likely due to the undercoordinated and rough surface with many potential

binding pockets. Furthermore, the HCOO* intermediate was found to optimize well on the

Bi(1,2,1) surface for all configurations tested. However, for the Bi(1,-1,0) surface, some starting

configurations lead to a decomposition of the HCOO* intermediate to CO*, O*, H*, and CO2.

Thermodynamic quantities for all surfaces are shown in Table S1.

29

Table S1. Calculated thermodynamic quantities of adsorbates on Bi slabs (eV).

Bi(1,-1,0)

Configuration Elec E_ZPE Cv_harm (0->T) -T*S G (eV) ΔG(ads) (eV)

Slab -163.54

H*-1 -166.67 0.15 0.02 -0.03 -166.53 0.50

H*-2 -166.32 0.15 0.02 -0.03 -166.18 0.84

H*-3 -167.02 0.14 0.02 -0.02 -166.88 0.14

H*-4 -166.60 0.14 0.02 -0.02 -166.48 0.55

H*-5 -166.94 0.15 0.02 -0.03 -166.80 0.23

H*-6 -166.67 0.15 0.02 -0.02 -166.53 0.50

HCOO*-1 Decomposed to CO* + O* + H*

HCOO*-2 -189.63 0.61 0.10 -0.20 -189.11 -0.01

HCOO*-3 -189.81 0.62 0.10 -0.19 -189.28 -0.18

HCOO*-4 Decomposed to CO2 + H*

Bi(1,2,1)

Configuration Elec E_ZPE Cv_harm (0->T) -T*S G (eV) ΔG(ads) (eV)

Slab -162.36

H*-1 -165.09 0.15 0.02 -0.03 -164.95 0.90

H*-2 -164.63 0.14 0.02 -0.03 -164.51 1.34

H*-3 -165.13 0.14 0.01 -0.02 -164.99 0.86

H*-4 -165.09 0.14 0.01 -0.02 -164.96 0.89

H*-5 -164.93 0.14 0.01 -0.02 -164.80 1.05

H*-6 -165.03 0.14 0.02 -0.02 -164.90 0.95

H*-7 -164.74 0.14 0.02 -0.02 -164.60 1.24

HCOO*-1 -187.63 0.60 0.11 -0.25 -187.17 0.76

HCOO*-2 -187.69 0.60 0.11 -0.23 -187.21 0.72

HCOO*-3 -187.53 0.59 0.12 -0.28 -187.10 0.82

HCOO*-4 -187.56 0.60 0.11 -0.23 -187.09 0.84

30

Effect of O* and OH*:

We also calculated the O* and OH* Gibbs free energy of adsorption on the Bi(1,-1,0) and

Bi(1,2,1) surfaces to provide further insights into how oxygen or hydroxide species may impact

the reactivity on the Bi surface. The additional calculations are displayed in Table S2. We find

that HCOO* is still more favoured over O* and OH*, suggesting that poisoning by OH* would

should not be rate limiting. The adsorption sites of O* and OH* were taken to be the same as

the previously calculated H* sites. Interestingly, it was found that for the Bi(1,-1,0) surface,

many of the adsorption sites for both O* and OH* were unstable with the starting configuration

failing to optimize. The Gibbs free energy of adsorption for all major intermediates are shown

in Figure S6 (averaged over all adsorption sites). It was found that generally on both Bi facets,

O* had very high ΔG(ads) of 1.24 eV and 1.57 eV for the (1,-1,0) and (1,2,1) facets

respectively. The OH* ΔG(ads) followed a similar trend to the HCOO* intermediate but were

still larger than HCOO* by ~0.1 eV, suggesting that while bound hydroxide is likely to be

present, the bound HCOO* is more favourable and thus Bi active sites are not actively poisoned.

Figure S6. The Gibbs free energy of adsorption for the H*, O*, OH* and HCOO* intermediates

on Bi(1,-1,0), and Bi(1,2,1) facets.

-0.50

0.00

0.50

1.00

1.50

2.00

H* O* OH* HCOO*

ΔG

(ads)

(ev)

Bi(1,-1,0) Bi(1,2,1)

31

Table S2. Thermodynamic quantities

Bi(1,-1,0)

Configuratio

n

Elec E_ZP

E

Cv_harm (0-

>T)

-T*S G (eV) ΔG(ads)

(eV)

Slab -163.54

H*-1 -166.67 0.15 0.02 -0.03 -166.53 0.50

H*-2 -166.32 0.15 0.02 -0.03 -166.18 0.84

H*-3 -167.02 0.14 0.02 -0.02 -166.88 0.14

H*-4 -166.60 0.14 0.02 -0.02 -166.48 0.55

H*-5 -166.94 0.15 0.02 -0.03 -166.80 0.23

H*-6 -166.67 0.15 0.02 -0.02 -166.53 0.50

O*-1 -

169.203

0.037 0.048 -

0.102

-

169.219

1.49

O*-2 Not Stable

O*-3 Not Stable

O*-4 Not Stable

O*-5 -169.49 0.038 0.048 -

0.101

-

169.505

1.21

O*-6 -

169.682

0.037 0.048 -0.1 -

169.697

1.02

OH*-1 Not Stable

OH*-2 Not Stable

OH*-3 -

174.209

0.036 0.049 -

0.102

-

174.226

-0.03

OH*-4 Not Stable

OH*-5 -

174.015

0.038 0.048 -

0.101

-174.03 0.17

OH*-6 -

173.886

0.037 0.048 -0.1 -

173.901

0.30

HCOO*-1 Decomposed to CO* + O* + H*

HCOO*-2 -189.63 0.61 0.10 -0.20 -189.11 -0.01

HCOO*-3 -189.81 0.62 0.10 -0.19 -189.28 -0.18

HCOO*-4 Decomposed to CO2 + H*

32

Bi(1,2,1)

Configuratio

n

Elec E_ZP

E

Cv_harm (0-

>T)

-T*S G (eV) ΔG(ads)

(eV)

Slab -162.36

H*-1 -165.09 0.15 0.02 -0.03 -164.95 0.90

H*-2 -164.63 0.14 0.02 -0.03 -164.51 1.34

H*-3 -165.13 0.14 0.01 -0.02 -164.99 0.86

H*-4 -165.09 0.14 0.01 -0.02 -164.96 0.89

H*-5 -164.93 0.14 0.01 -0.02 -164.80 1.05

H*-6 -165.03 0.14 0.02 -0.02 -164.90 0.95

H*-7 -164.74 0.14 0.02 -0.02 -164.60 1.24

O*-1 -168.09 0.15 0.02 -0.03 -167.95 1.59

O*-2 -168.02 0.03 0.05 -0.11 -168.05 1.49

O*-3 -167.97 0.04 0.05 -0.10 -167.98 1.56

O*-4 -168.03 0.04 0.05 -0.10 -168.04 1.49

O*-5 -167.71 0.03 0.05 -0.10 -167.72 1.81

O*-6 -168.03 0.04 0.05 -0.10 -168.05 1.49

OH*-1 Not Stable

OH*-2 -

172.119

0.034 0.05 -

0.111

-

172.146

0.88

OH*-3 -

172.119

0.035 0.05 -

0.096

-172.13 0.89

OH*-4 -

172.101

0.035 0.049 -

0.097

-

172.114

0.91

OH*-5 -

171.899

0.034 0.049 -

0.098

-

171.914

1.11

OH*-6 -

171.931

0.034 0.049 -

0.098

-

171.946

1.08

HCOO*-1 -187.63 0.60 0.11 -0.25 -187.17 0.76

HCOO*-2 -187.69 0.60 0.11 -0.23 -187.21 0.72

HCOO*-3 -187.53 0.59 0.12 -0.28 -187.10 0.82

HCOO*-4 -187.56 0.60 0.11 -0.23 -187.09 0.84

33

Sample fabrication

Chemicals: All chemicals were purchased from Sigma Aldrich. Carbon paper substrates (Toray

TGP-H-060 and Freudenberg for H-cell and flow-cell respectively) were purchased from Fuel

Cell Store. The standard size of the substrates for H-cell measurements was 0.1–0.3 cm2. The

size of the substrates employed in the flow-cell configuration was 4 cm2.

Film deposition: BiBr3 was dissolved in DMSO at concentrations at a 1 M concentration. The

solution was vortexed until all the salts were dissolved. The solution was drop-casted on top of

the carbon substrates inside a glovebox with a N2 atmosphere at around 250 µL/cm2. Samples

were heated up to 140°C until the solvent fully evaporated. Samples on Freudenberg gas

diffusion electrode were deposited after 5 min of plasma treatment by blade coating, and the

samples heated up to 90°C until the solvent fully evaporated.

Sample conditioning: after immersion in a CO2 saturated 1 M KHCO3 electrolyte samples and

the samples were hydrated they turned white. Cyclic voltammetry (CV) measurements at 50

mV s-1 were performed for 10 cycles for sample conditioning from -0.6 V vs Ag/AgCl to -2 V

vs. Ag/AgCl until the sample acquired a black metallic color.

Material characterization

Sransmission Electron Micrograph (SEM): SEM images were obtained by applying a 10kV and

20 uA using a Hitachi S-5200 apparatus available in The Centre for Nanostructure Imaging,

University of Toronto.

High-Resolution Transission Electron Micrograph (HR-TEM): For transmission electron

microscopy measurements, a 300 kV Hitachi HF-3300 was utilized. The d spacing was

characterized using Gatan microscopy suite software.

34

X-ray diffraction (XRD): patterns were obtained with Rigaku MiniFlex600 diffractometer

using monochromatic Copper Kα radiation (λ = 1.5406 Å). Data were collected in Bragg-

Brettano mode using 0.03° divergence with a scan rate of 0.05° s-1.

X-ray Photoemission Spectroscopy (XPS): XPS measurements were carried out using a Thermo

Scientific K-Alpha system, with a 75 eV pass energy, and binding energy steps of 0.05 eV.

X-ray absorption and scattering: X-ray absorption measurements at the Bi L-edges were

performed at the 06ID-1 multipurpose hard X-ray beamline, based on a 63 pole superconducting

wiggler. The window of the sample cells was mounted at an angle of roughly 45° with respect

to both the incident beam and the detectors. All measurements were made at room temperature

in the fluorescence mode using Amptek silicon drift detectors (SDDs) with 1024 emission

channels (energy resolution ∼120 eV). Four SDDs were employed simultaneously. For every

edge, the scanning time was 30 s and repeated for ten times, and the fluorescence of every edge

was collected at the same absorption edge. The partial fluorescence yield (PFY) was extracted

from all SDDs by summation of the corresponding metal L emission lines. Grazing Incident

Wide-Angle X-ray Spectroscopy (GIWAXS) were performed in a homemade cell at a 0.5°

inclination.

Electrochemical characterization

Electrochemical measurements: were performed using a three-electrode system connected to

an electrochemical workstation (Autolab PGSTAT302N) with built-in electrochemical

impedance spectroscopy (EIS) analyzer. Ag/AgCl (with saturated KCl as the filling solution)

and platinum foil were used as reference and counter electrodes, respectively.

Electrode potentials were converted to the reversible hydrogen electrode (RHE) reference scale

using

E(RHE) = E(Ag/AgCl) + 0.197 V + 0.0591×pH

35

The calibrations of Ag/AgCl reference electrode and Ag wire reference electrode were

conducted in the standard three-electrode system (the same system as that for performance

measurements) as reference electrodes, using Pt foil as working and counter electrodes. The

electrolyte was 0.1 M KHCO3 saturated with CO2. Formate was quantified on GC with a mass

spectrometry (PerkinElmer Clarus 600 GC-MS System). Assuming that two electrons are

needed to produce one formate molecule, the Faradaic efficiency was calculated as follows:

F.E. = 2 × F × nformate / Q = 2 × F × nformate / (I × t),

where F is the faraday constant, I is the current, t is the running time and nformate is the total

amount of produced formate (in moles).

iR correction: we carried out iR correction to account for the cell resistance. The cell resistance

was determined by electrochemical impedance spectroscopy to be (Rs = 45Ω for H-cell, 8Ω for

flow-cell), in line with previous reports. At all potentials tested, the potential was manually

corrected using Ohm's law:

E = Eapp – I × Rs × 0.90,

(compensating for 90% of the resistance) where Rs represents the calculated solution resistance,

I is the measured current, and 𝐸app is the applied potential on the working electrode

Product characterization

Gas products analysis: gas products were analyzed using a gas chromatograph (PerkinElmer

Clarus 680) coupled with a thermal conductivity detector (TCD) and a flame ionization detector

(FID). The gas chromatograph was equipped with a Molecular Sieve 5A capillary column and

a packed Carboxen-1000 column. Argon (Linde, 99.999%) was used as the carrier gas.

36

Liquid products analysis: liquid products were analyzed using nuclear magnetic resonance

spectroscopy (NMR). 1H NMR spectra of the acquired samples were collected on Agilent DD2

500 spectrometer (available in the Department of Chemistry at the University of Toronto) in

10% D2O using water suppression mode, with Dimethyl Sulfoxide (DMSO) as an internal

standard. Thirty two second relaxation time between the pulses was used to allow for complete

proton relaxation. The reported results are the average during the reaction time.

Figure S5. Representative nuclear magnetic resonance (NMR) spectra of the liquid

products

37

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