advances.sciencemag.org/cgi/content/full/3/8/e1700686/DC1

Supplementary Materials for

Biofunctionalized conductive polymers enable efficient

CO2 electroreduction

Halime Coskun, Abdalaziz Aljabour, Phil De Luna, Dominik Farka, Theresia Greunz, David Stifter,

Mahmut Kus, Xueli Zheng, Min Liu, Achim W. Hassel, Wolfgang Schöfberger,

Edward H. Sargent, Niyazi Serdar Sariciftci, Philipp Stadler

Published 4 August 2017, Sci. Adv. 3, e1700686 (2017)

DOI: 10.1126/sciadv.1700686

This PDF file includes:

fig. S1. Window of redox stability of conductive PDA.

fig. S2. Electrochemical setup for electrocatalytic CO2RR.

fig. S3. Local pH as measured in the electrolyte system (0.1 M TBA-PF6, 25°C,

acetonitrile, CO2-saturated) at various amounts of water added.

fig. S4. 13C NMR spectra.

fig. S5. Conductivity versus time.

fig. S6. Conductivity (dc) of PDA in various liquids.

fig. S7. pH values during electrosynthesis of formate in acetonitrile-water (0.1 M

TBA-PF6) blends at 25°C, CO2-purged (30 min).

fig. S8. Chronoamperometric scan (continuous and semicontinuous).

fig. S9. Stability aspects and mechanisms shown by in situ FTIR.

fig. S10. Differential in situ spectra in the spectral fingerprint regime.

fig. S11. ATR-FTIR (in situ measurement cell) with reference 0.1 M TBA-

formate and saturated CO2.

fig. S12. XPS survey scan of conductive PDA.

fig. S13. N1s HR XPS scan.

fig. S14. C1s HR XPS scan.

fig. S15. O1s HR XPS scan.

fig. S16. O1s HR XPS scan.

table S1. State-of-the-art CO2RR electrocatalysts, namely, for CO, formate, and

related (hydro)carbon products.

table S2. Electrochemical impedance data.

Supplementary Materials

We determined the electrochemical stability of conductive PDA as synthesized by deposition

of a thin film (approximately 0.5 to 1 µm) onto FTO substrates. The electrochemical system

employed was:

- 0.1 M TBA-PF6 in acetonitrile with 1 vol% H2O added in an inert glovebox atmosphere

with O2 < 1 ppm;

- N2-purged solution for the control scans without presence of O2 and CO2;

- CO2-purged solution (at least 30 min vigorous purging) for electrocatalytic scans

(yielding 0.27 mol L-1 CO2 dissolved in acetonitrile-water at 25°C and ambient

pressure).

This system was studied electrochemically to determine the potential products, the

electrochemical potential window and the quasi-reference electrode calibration. We

conducted a speed study to assign the displacements currents and the catalytic activity, and

performed before-after scans to determine possible degradation mechanisms. The results

are shown in this order (fig. S1).

Calibration of QRE (fig. S1d): Note that the calibration versus the ferrocene-ferrocenium

couple yielded a reference potential as high as +400 mV (E1/2) vs. Ag/AgCl QRE, and

+640 mV vs. normal hydrogen electrode (NHE). All potentials (unless otherwise stated) are

given relative to NHE.

Stability of conductive PDA (fig. S1b): We performed an electron-voltage spectroscopy

scan (infinitesimally slow cyclic voltammogram to reduce the kinetic contribution). The step

onset in the charge can be seen in the diagram below -2000 mV vs. NHE. We therefore set

the upper experimental limit to -1800 mV vs. NHE.

fig. S1. Window of redox stability of conductive PDA. We examined the electrochemical

stability as derived (a) by cyclic voltammetry (CV) and (b) by electron-voltage-spectroscopy

(cyclic voltammogram under quasi-DC conditions) using pulsed steps between 0 and -

1800 mV vs. NHE (reductive side). Throuhgout we used acetonitrile and 0.1 M TBA-PF6

referenced versus an Ag/AgCl quasi-reference electrode (QRE). The system was calibrated

versus the ferrocene/ferrocenium couple (d) to define the offset between Ag/AgCl QRE and

NHE (ΔE = 240 mV). Further, we tested the speed of response to the CVs (c) and the

catalytic activity of conductive PDA (on FTO) and performed before and after CV-scans (f) to

demonstrate the reversibility of conductive PDA on the reductive side.

fig. S2. Electrochemical setup for electrocatalytic CO2RR. A two-component H-

cell was flushed with CO2 (0.1 M TBA-PF6, acetonitrile-water as electrolyte). The

glass frit separated anode and cathode space. We used a calibrated Ag/AgCl quasi-

reference electrodes (QRE) as reference electrode (RE), conductive PDA on a

carbon felt network as working electrode (WE), and platinum as counter electrode.

This setting allowed continuous operation of CO2RR. The products we observed

were formate, CO, H2 (cathode space) and O2 (anode space). The Faradaic

efficiency (FE) on the cathode side was almost quantitative (95-97% Faradaic

yields).

Determining the overpotential of CO (or formate) evolution:

We calculated the overpotentials according to the following equations, based on

quantum-chemical calculations for the reduction of CO2 to CO in a similar system

(0.1 M TBA-PF6 in acetonitrile-water, 5%vol or 1:4 acetonitrile to water):9

𝐸𝐶𝑂,𝐶𝑂2,𝐶𝐻3𝐶𝑁0 = 0.349 −

𝑅𝑇∙ln 10

𝐹∙ 𝑝𝐾𝑎 𝑉 vs. NHE

To determine the pKa, we considered the strongest acid in the system, i.e., CO2-H2O with a

pKa of 17.03. Hence, the standard potential for the reduction of CO in the presented

electrochemical setting was

𝐸𝐶𝑂,𝐶𝑂2,𝐶𝐻3𝐶𝑁0 = −650𝑚𝑉 vs. NHE

Previous investigations using acetonitrile as main solvent confirm this potential. The

overpotential of the presented CO evolution was calculated according to

𝜂𝐶𝑂 = 𝐸𝐶𝑂,𝐶𝑂2,𝐶𝐻3𝐶𝑁 − 𝐸𝐶𝑂,𝐶𝑂2,𝐶𝐻3𝐶𝑁0 = (−860𝑚𝑉) − (−650𝑚𝑉) = −210𝑚𝑉

Note that similar considerations for the competing formation of formate yield a similar

overpotential (according to a conservative estimation of the activity of protons).

Alternative determination of overpotential of CO (or formate) evolution

For practical comparison we sougth to use RHE as reference electrode (instead of NHE).

However, RHE needs careful evaluation of the pH, which is difficult in a predominantly

aprodic (and multi-component) solvent system. We measured the pH in the actual electrolyte

solution (CO2-saturated acetonitrile-x%water, 0.1 M TBA-PF6) from 1 to 5% water (added).

We found, that the pH values are vague (significant deviation). We report also a trend

towards lower pH upon increasing the water content. At 1% (actual electrolyte solution used

for CO2RR) the pH is found at average 7.9.

fig. S3. Local pH as measured in the electrolyte system (0.1 M TBA-PF6, 25°C,

acetonitrile, CO2-saturated) at various amounts of water added.

From the pH-measurements, we obtain an estimation of RHE vs. NHE for this particular

complex electrochemical system. Note we have just considered the initial values of pH (since

the error bar is significant and in addition, tend to increase by electrolysis time). However, we

consider this method less precise (as compared to the method used in the manuscript), since

acetonitrile is considered as dominant (aprotic) solvent. Using averaged pH (across time i.e.

pH = 7.9) we apply equation (i) as followed

𝑅𝐻𝐸 = 𝑁𝐻𝐸 − 0.059 ∙ 𝑝𝐻 (i)

and plot vs. RHE (potential shifts by -470mV). Thus -860 mV vs. NHE (applied operating

potential for the chronoamperometric scans) translates to -390 mV vs. RHE.

We add a comparative survey from recent literature on similar electrocatalytic systems (state-

of-the-art, May 2017)

table S1. State-of-the-art CO2RR electrocatalysts, namely, for CO, formate, and related

(hydro)carbon products. *Covalent organic frameworks (cobalt-porphyrine); †nanoparticles

of Ag; ‡Palladium nanoparticles; #as referred to indicated overpotential.

Determining the turnover frequency

To calculate the turnover, we have determined the weight of PDA on carbon felt. We refer

the effective turnover number to polydopamine´s average monomer (average molecular

weight is 168 g mol-1). E.g. we found 1.03 mg PDA on a typical CF+PDA electrode. We

calculate the turnover as followed:

In total, we observe 720 ppm formate after 16 h electrolysis in 10 ml solution according to the

chronoamperometric scan presented in Fig. 4d. This corresponds to an absolute amount of

7.2 mg formate in 10 ml solution (0.72 mg ml-1 i.e. 16mM solution formate). The catalyst

activity is hence calculated as followed

𝑇𝑂𝑁𝑝𝑒𝑟 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑚𝑜𝑛𝑜𝑚𝑒𝑟 =𝑚𝑜𝑙 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑝𝑟𝑜𝑑𝑢𝑐𝑡

𝑚𝑜𝑙 𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡=

1.6∙10−4𝑚𝑜𝑙𝑚𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡

𝑀𝑎𝑣. 𝑚𝑜𝑛𝑜𝑚𝑒𝑟=6.1∙10−6𝑚𝑜𝑙

∙ 𝐹𝐸𝑓𝑜𝑟𝑚𝑎𝑡𝑒 = 19.7 (ii)

𝑇𝑂𝐹𝑝𝑒𝑟 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑚𝑜𝑛𝑜𝑚𝑒𝑟 = 19.7

16ℎ= 1.23 ℎ−1 (iii)

13C NMR for exact product analysis

To address the fact, that we have multiple latent carbon-resources besides CO2 (carbon felt

electrode, acetonitrile solvent, tetrabutylammonium-cations) we conduct a cross-check of the

origin of electrocatalyzed formate. Therefore we perform separate chronoamperometry and

spectral 13C NMR using marked 13CO2. For the setup, we use a deuterated electrolyte

solution (CD3CN and 100 ppm D2O). We ran a sequence (chronoamperometry at -1100 mV

vs. Ag/AgCl) for 2 hours and record spectra from an product-electrolyte sample (450 µl).

fig. S4. 13C NMR spectra. (a) 13C NMR spectra showing marked 13CO2 (acetonitrile-water,

chemical shift δ = 124.8 ppm), trace amount oxalate (157.7 ppm), formate (166.7 - 167.0 ppm)

and trace amount carbon monoxide (170.8 ppm). Note the 13C-splitting of deuterated formate

(actually DCOO-) leading to broader peaks. (b) Zoom to D-splitted formate (D2O-presence)

providing additional evidence that the product is derived from CO2 (and D2O).

13C NMR spectra shows three signatures of oxalate (by-product of formate coupling, trace

amount), formate (main product) and CO. The zoom in the formate peak reveals D-splitting

(which confirmed that the product i.e. deuterated formate originates from 13CO2 and D2O).

Note the 13C NMR spectra are evaluated only qualitatively.

Electrical conductivity

fig. S5. Conductivity versus time. 4-wire measurement (4 contacts in line) examining the

bulk conductivity as a function of temperature (see Fig. 2) and time (seen here). Note the

stability of the electrical parameters over a 17 h run (1000 min). The photograph (inset)

shows the thin film contacted onto the testing rack (Dynacool PPMS).

In addition to time dependent conductivity (fig. S5) we have explored the conductivity of PDA

immersed in various liquids to explore the impact on conductivity (ionic contribution) (fig. S6)

such as

- deionized water

- acidic (pH 4) and neutral (buffered pH 7) water

- acetonitrile, 1% water, 0.1 M TBA-PF6 (electrolyte-solution)

fig. S6. Conductivity (dc) of PDA in various liquids. We observe similar values in aqueous

surrounding (i.e. H2O, pH = 4 and pH = 7 (buffer) and acetonitrile solutions (0.1 M TBA-PF6).

We find the conductivity is not highly impacted by the surrounding media. The conductivity

(0.4-0.6 S cm-1) is hence considered as truly electronic (as furthermore backed by the

spectroscopic fingerprint signatures such as IRAV-bands).

Impedance measurements for iR

Continuative impedance measurements show that the resistance of PDA catalyst on carbon

felt is negligible. We conducted impedances (i.e. iR) values in the actual electrolysis cells, to

show the cell parameters accordingly

i. for the “naked” cell (Pt working electrode, Pt counter electrode, 2-electrode

configuration) to explore the resistance of the cell (incl. glass frit) in 0.1 TBA-PF6 in

acetonitrile + 1% water and CO2 saturated.

ii. for (inactivated) bare carbon-felt (CF, CF as working electrode, Pt as counter

electrode, 2-electrode configuration)

iii. for activated polydopamine-CF (vs. Ag/AgCl, Pt as counter electrode).

table S2. Electrochemical impedance data.

In addition we explore the uncompensated iR (actual resistance of the electrolyte without frit,

static or kinetic losses) in 3-electrode configuration (i.e. the actual electrolysis cell). We find a

resistance of 65 Ohms (between 1 MHz and 1 kHz) and 129 Ohms (between 1 kHz and 1 Hz).

Chronoamperometric details – continuous and discontinuous electrosynthesis

We found the changes in current correlate to an increase in pH (i.e. the product yield of

catalysis is shifted from CO/H2 to formate). We measured the pH at beginning of the

electrolysis to be 7.9 ± 1). Note that certainty of the measurement is rather low in acetonitrile.

During our semi-continuous electrolysis, we see a trend to higher pH (presence of formate);

i.e. the liquid product yield is significantly large (16 mM) to alter the concentration of the

electrolyte.

fig. S7. pH values during electrosynthesis of formate in acetonitrile-water (0.1 M TBA-

PF6) blends at 25°C, CO2-purged (30 min).

Working electrode Frequency Configuration iR (Ohms) Counter electrode

Platinum (ref.) 1 kHz to 1 MHz 2 electrodes 1360 Pt

Carbon felt (CF) 1 kHz to 1 MHz 2 electrodes 857 Pt

Polydopa

mine-

carbon felt

(PDA-CF)

1 kHz to 1 MHz 3 electrodes* 907

741

Pt

*Ag/AgCl QRE

The semi-continuous setup is responsible for the increase in current, since we change the

ionic strength by electrolysis (producing formate). This will increase the conductivity. We

present a calibration scan at the same conditions under constant potential with replacing the

solution more frequently (per 3-4 h), where we observe constant currents. Product

accumulation could be resolved by using continuous extraction of formate from the

electrolyte solution.

fig. S8. Chronoamperometric scan (continuous and semicontinuous). Frequent

exchange of the electrolyte solution leads to a constant current vs. time characteristic.

In-situ spectroelectrochemistry - details

fig. S9. Stability aspects and mechanisms shown by in situ FTIR. (a) electrochemical cell

optimized for the in-situ detection of conductive PDA as electrode. (b) before-after spectra

indicating the reversibility of conductive PDA electrocatalysis and showing adsorption of

minor amounts of TBA+, acetonitrile and PF6-. In the presence of CO2, residual adsorption of

CO2 to conductive PDA and corresponding changes in the carbonyl functions are observed.

(c) and (d) show individual in-situ difference spectra between 0 and -1500 mV vs. NHE. No

conductive PDA-related changes are observed in the absence of CO2. In the presence of

CO2, catalytic activity is visible, particularly a decrease in the carbonyl function and a

corresponding adsorption of CO2.

In-situ spectroelectrochemistry (isSEC) is a powerful tool for examining processes on the

surface of an electrocatalyst. This method has been used extensively to probe the doping in

conductive polymers in the fingerprint regime in the near- and mid-infrared fingerprint

spectral regime between 4000 and 400 cm-1. Our setup consisted of a flow cell with the

electrolyte system, WE (germanium and conductive PDA as deposited), QRE and Pt-CE fig.

S9a). The germanium had two functions: as a transparent carrier electrode, as a reflection

element with a refractive index (nr = 4 at 2000 cm-1) higher than that of the adjacent layer of

conductive PDA (nr for organics is typically 1.5 to 2) for total reflection (and the

corresponding evanescent wave passing through conductive PDA to the surface and the

adjacent electrolyte layer). The changes we observed in the spectra are related to changes

in potential (differential scans). Our goal was to visualize the processes that emerge directly

from the electrocatalytic reduction of CO2. The before-after scans with and without CO2

underline the reversibility of the discussed reactions (fig. S9b): PDA itself does not exhibit

spectral changes (as previously found in the CV scans). The features seen in the before-after

scans relate either to the enrichment or to the depletion of the electrolyte and to solvent

uptake or, in the presence of CO2, its accumulation at the electrode surface (as indicated in

the figure). The corresponding isSEC-spectra of N2-purged or CO2-purged electrolyte at

maximum reduction potential -1560 mV vs. NHE (figs. S9c and S9d) highlights the CO2RR-

related features (i.e., CO2 adsorption and carbonyl features), which emerge only in the

presence of CO2. Note that for N2-purged electrolyte, the spectrum had no relevant catalytic

features (except the previously stated changes in the electrolyte salt or solvent penetration).

A potential-dependent multiple differential scan is also included to illustrate the emergence of

catalytic features (fig. S10). In addition we included a scan with stock chemicals (formate and

CO2) for comparison (fig. S11).

fig. S10. Differential in situ spectra in the spectral fingerprint regime. The differential

spectra show only the changes in the spectral features upon increasing the cathode potential

(vs. NHE). Upwards-pointing changes indicate the emergence of new features, such as

accumulation of TBA+ at the electrode, adsorption of CO2 and new amide-type carbonyl

features. Downwards-pointing changes reflect the depletion of PF6- and the reduction of the

main carbonyl function on conductive PDA. The differential spectral survey shown here

complement Fig. 3 in the main text.

fig. S11. ATR-FTIR (in situ measurement cell) with reference 0.1 M TBA-formate and

saturated CO2.

fig. S12. XPS survey scan of conductive PDA. The elemental concentration was

determined based on the survey scan and subsequent high-resolution scans (particularly

C1s, N1s, O1s and S2p).

fig. S13. N1s HR XPS scan. The nitrogen analysis offers insight into the structure of

conductive PDA. Primary amine (-NH2) is dominant in the structure (68.6 %), which is

reflected in the synthesis path (cf. Fig. 2).

fig. S14. C1s HR XPS scan. The carbon peak shows the different structural moieties in

conductive PDA – these are CHx, C-O and C-N or carbonyl and the conjugated core.

fig. S15. O1s HR XPS scan. The oxygen peaks indicate the environment either related to

sulfate anions or the dopamine ring (including the carbonyl- and hydroxyl fractions).

fig. S16. O1s HR XPS scan. Sulfur in conductive PDA is related to the presence of bisulfate

(and/or sulfate).