Optimizing FeNC Materials as Electrocatalysts for the CO2 Reduction Reaction: Heat-Treatment Temperature, Structure and Performance Correlations

Article abstract of DOI:10.1002/cctc.201901196

The direct CO₂ electrochemical reduction reaction (CO2RR) into carbon-based chemicals has attracted tremendous attention as a sustainable process for CO₂ utilization. The viability of the process, however, is contingent on finding efficient catalysts based on earth abundant elements. Carbon-based solid catalyst materials doped with nitrogen and transition metals (M?N?C) have emerged as a cost-efficient alternative for the direct electrochemical reduction of CO₂ into CO. These materials contain different N functionalities and MN_x moieties which could be involved in the catalytic process. With the aim of gaining insight into the role of these different active sites and how to control their concentration we have prepared 5 polyaniline derived FeNC catalysts by changing the temperature of the heat treatment (750–1050 °C). We observed that it is possible to tune the ratio of the different N functionalities by changing the pyrolysis temperature. Furthermore, this had a clear impact on the catalytic performance of this family of FeNC materials. In particular, a higher temperature correlated with a larger FeN_x concentration resulting in a high selectivity toward the CO2RR. These results indicate that FeN_x moieties play a predominant role in the catalytic process and that the incorporation of such sites to the carbon structure is enhanced by the heat-treatment temperature.

Full text of DOI:10.1002/cctc.201901196

Accepted Article
Title: Optimizing FeNC materials as electrocatalyst for the CO2
reduction reaction: Heat treatment temperature, structure and
performance correlations
Authors: Eduardo González-Cervantes, Anai Arellano Crisóstomo,
Aída Gutiérrez-Alejandre, and Ana Sofia Varela
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201901196
Link to VoR: http://dx.doi.org/10.1002/cctc.201901196
A Journal of
www.chemcatchem.org

10.1002/cctc.201901196
ChemCatChem
Optimizing FeNC materials as electrocatalyst for the CO2 reduction
reaction: Heat treatment temperature, structure and performance
correlations
Eduardo González-Cervantesa Anaí Arellano Crisóstomoa, Aída Gutiérrez-Alejandreb, Ana Sofía
Varelaa*
aInstituto de Química, Universidad Nacional Autónoma de México, Coyoacán, 04510, Ciudad de
México, Mexico.
bUNICAT, Departamento de Ingeniería Química, Facultad de Química, Universidad Nacional
Autónoma de México, Coyoacán, 04510, Ciudad de México, Mexico.
*Corresponding author:
Ana Sofia Varela, email: asvarela@iquimica.unam.mx
Abstract
The direct CO2 electrochemical reduction reaction (CO2RR) into carbon-based chemicals has
attracted tremendous attention as a sustainable process for CO2 utilization. The viability of the
process, however, is contingent on finding efficient catalysts based on earth abundant elements.
Carbon-based solid catalyst materials doped with nitrogen and transition metals (M-N-C) have
emerged as a cost-efficient alternative for the direct electrochemical reduction of CO2 into CO.
These materials contain different N functionalities and MNx moieties which could be involved in the
catalytic process. With the aim of gaining insight into the role of these different active sites and how
to control their concentration we have prepared 5 polyaniline derived FeNC catalysts by changing
the temperature of the heat treatment (750-1050°C). We observed that it is possible to tune the
ratio of the different N functionalities by changing the pyrolysis temperature. Furthermore, this had
a clear impact on the catalytic performance of this family of FeNC materials. In particular, a higher
temperature correlated with a larger FeNx concentration resulting in a high selectivity toward the
CO2RR. These results indicate that FeNx moieties play a predominant role in the catalytic process
and that the incorporation of such sites to the carbon structure is enhanced by the heat treatment
temperature.
1
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901196
ChemCatChem
The catalytic performance polyaniline derived FeNC catalysts towards the CO2 reduction reaction
can be modified by changing the temperature of the heat treatment during the material synthesis.
The catalyst treated at the lowest temperature is the most active one duet to its large
electrochemical surface area. The selectivity towards the CO2RR, however, is enhanced with a
higher synthesis temperature, which is attribute to large concentration of FeNx, highlighting the
importance of such sites on the catalytic performance of MNC materials.
Introduction
The CO2 electrochemical reduction reaction (CO2RR) is a promising technology for converting waste
CO2 into chemicals as it is a process that can contribute to mitigate climate change on two fronts.1
On one hand, it is a sustainable option for CO2 utilization converting it into carbon-based chemicals
and fuels. One the other hand, this process could utilize the surplus electricity, produced from
intermittent sources such as wind and solar power, avoiding wasting excess of electricity generated
during the peaks of production.2
The efficiency of the process relies heavily on the material used as electrocatalyst for the CO2RR.
Metallic or partially oxidized copper is usually the catalyst of choice given its unique ability to reduce
CO2 into hydrocarbons.3 This process, however, suffers from poor selectivity since it is accompanied
by hydrogen evolution reaction (HER). In addition, CO2 is reduced to different products mainly CH4,
C2H4, CO and formic acid, which hinders the selective formation of one particular product.3-8
Furthermore, the reaction takes place at high overpotentials which implies important energy loses.
Most of these losses can be attributed to the multiple proton/electron transfer steps involved in
hydrocarbon production. By contrast, the direct electroreduction of CO2 into CO is a less hindered
process as it involves the transfer of only two electrons and two protons, according to the following
reaction:
CO2 + 2H+ + 2e− → CO + H2O
2
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901196
ChemCatChem
The formation of CO is usually accompanied by HER resulting in a mixture of H2 and CO which in the
right proportion, can be used for synthetic fuels production via the catalytic Fischer-Tropsch
process.9 Pure CO streams, however, are also desired for other processes such as the synthesis
phosgene for polyurethane and polycarbonate production.
The first heterogeneous catalysts reported to selectively reduce CO2 into CO were based on precious
metals such as silver and gold on which CO is formed at low overpotentials with high faradaic
efficiencies.10-12 Given their low abundance, these materials are not optimal for developing
affordable electrolyzers. Therefore, there is an increasing interest in developing CO2RR catalysts
based on earth-abundant elements.
Recently, carbon-based solid catalyst materials doped with nitrogen and nitrogen-coordinated
transition metals (M-N-C) have emerged as a selective and cost-efficient alternative to noble metal
catalysts for the direct electrochemical reduction of CO2 to CO.13-18 Such materials have been widely
studied as an alternative to Pt catalyst for the Oxygen Reduction Reaction (ORR) in cathode’s fuel
cells.19-22 To explore the possibility of using M-N-C catalyst for the CO2RR Tripkovik et al., performed
Density Functional Theory calculations (DFT) on porphyrin-like metal-functionalized graphene
structures and found that these materials should in fact be active in the reduction of CO2.23 Later,
Varela et al. reported that polyaniline (PANI) derived carbon materials exhibit a high selectivity
towards CO production with a faradaic selectivity close to 80% at low overpotential (–0.55V vs RHE).
Interestingly, they observed high CO selectivity in the presence and absence of a metal center (Fe
or Mn).13 The activity, however, was dramatically increased by incorporation of a metal to the N-C
structure. Indicating that, despite some activity of the N functionalities, the activity of the metal
center is dominant. Furthermore, the metal center has been shown to have a crucial role on
determining the catalytic performance.13,14 In particular, FeNC catalysts present a low onset
potential for CO, which has been proposed to be formed via a decoupled proton electron transfer
-
to form (FeNC)-CO2 as the key intermediate, followed by an electron transfer to form COOH which
then can be reduced to CO and water.24 Combined DFT and experimental studies have found that
the high binding energy of the FeNx centers towards CO and CO2 is responsible for the early CO
onset potential on FeNC catalyst.15 This high binding energy is also responsible for the further
reduction of CO to CH4 which has also been observed as a minor product.
In addition to the nature of the metal center, there are other structural factors that can affect the
catalytic activity of MNC materials, such as the coordination environment of the metal center,25,26
the presence of different N functionalities and the carbon structure,27-29 revealing the complexity of
3
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901196
ChemCatChem
these catalysts. Such structural parameters can be affected by the synthesis methods, as it was
published in the work by Leonard et al. which showed that the addition of a secondary N precursor
affects the ratio of the different N functionalities present in polyaniline based FeNC material.27 For
instance, the material prepared with melamine resulted in higher concentration of Fe–N moieties,
while the material synthesized using nicarbazin revealed the highest concentration of N-Ox. This
difference in composition also resulted in a change in performance toward the CO2RR. Interestingly,
when correlating the performance towards the CO2RR with the catalyst composition, it was found
that Fe–N moieties are not the only functionalities involved in the catalytic process and that also
pyridinic nitrogen is playing a major role on the reduction of CO2, indicating that there are different
possible active sites present in these catalysts.
Herein, we explore the effect of the synthesis heat treatment temperature on the catalysts structure
and how it affects its performance towards the CO2RR. In particular, we have prepared 5 polyaniline
derived FeNC catalysts changing the temperature of the heat treatment (750-1050°C). The
temperature of the heat treatment has been shown to have an effect on performance of MNC
catalysts toward the ORR,30,31 but its effect towards the CO2RR had not been explore. In this work,
we show that the heat treatment temperature affects both the electrochemical surface area and
the ratio between the different N functionalities, resulting in a distinct catalytic activity for each of
the 5 tested materials. We found that a high micropore area enhances the catalytic activity of this
family of materials. The selectivity, however, depends on the nature of the nitrogen incorporated
to the material. A larger concentration of FeNx moieties, results in a higher CO2RR selectivity. In
particular, we highlight the importance of FeNx moieties which serves as highly selective site for the
selectively reducing CO2. These results contribute to gain a better understanding on the catalytic
performance of FeNC catalysts towards the CO2RR.
Results and discussion
A range of FeNC catalysts were synthesized starting from FeCl3, polyaniline (PANI), Ketjen EC 600JD
(AzkoNobel) by varying the synthesis temperature (750, 825, 900, 975 and 1050°C). X-Ray Diffraction
(XRD) patterns for the 5 catalysts are shown in Figure S1, showing an amorphous character of the
materials. The two broad signals at 2θ = 24.5°and 2θ = 43.5° can be attributed to the (002) and (100)
graphite planes. In addition, residual crystalline inorganic FexSy species, namely pyrrhotite and
pyrite, can be detected on the catalysts prepared at higher temperatures. The presence of such
inorganic particles was corroborated by Transmission Electron Microscopy images (Figure S2).
Similar to previous works,30,32 we observed that on the samples prepare at temperatures ≥900°C
4
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901196
ChemCatChem
there are nanoparticles embedded in the carbon structure, which makes them difficult to be
removed via acid leaching. In addition, the microscopy images in Figure 2 show the amorphous
structure of the carbon materials, in which the carbon layers can be distinguish. This, however, is
not the case for the FeNC 1050 indicating that the carbon structure is clearly modified when treated
at such high temperatures.
The composition of the resulting materials according to elemental analysis and X‐ray photoemission
spectroscopy (XPS) is given in table 1. Consistent with previous results, the N content diminishes
with the heat treatment while the Fe concentration does not show a clear correlation to the heat
treatment temperature. The N1s peak revealed the presence of a variety of nitrogen species (Figure
1) including the FeNx confirming that iron has been incorporated to carbon coordinating through
nitrogen. Such MNx have been proposed as the predominant active site of MNC catalysts during the
CO2RR.33,34 The ratio between such N species was found to be dependent on the synthesis
temperature, as shown in table 1. As one can expect, a higher temperature results in large
graphitization of the MNC materials and thus the ratio of graphitic N increases with temperature,
while the concentration of pyridinic N decreases. In addition, the concentration of FeNx increases
with the temperature of the heat treatment during the catalysts synthesis indicating that a higher
concentration of active sites can be incorporated when treating the materials at elevated
temperatures.
Figure 1: High resolution N1s XPS signal of the different Fe-N-C materials showing the contribution
5
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901196
ChemCatChem
of the different N functionalities a) FeNC 750, b) FeNC 825 c) FeNC 900 d) FeNC 975 and e)FeNC
1050.
Table 1: Elemental composition according to XPS and elemental analysis.
Concentration obtained by elemental analysis (%)
Element
FeNC 750
66.33
11.12
0.57
FeNC 825
74.41
7.19
FeNC 900
67.32
4.32
FeNC 975
78.47
3.82
FeNC 1050
84.88
2.75
C
N
H
S
0.98
0.44
0.44
0.44
6.19
1.36
5.50
2.97
2.42
Concentrations obtained by XPS (%) (from figure S3)
Fe 2P3/2 4.88 5.20
4.40
4.56
4.71
Relative concentration of N functionalities for each catalyst (%) (From figure 1)
N functionality FeNC 750
FeNC 825
25.96
FeNC 900
24.87
14.04
44.81
11.51
4.75
FeNC 975
9.92
FeNC 1050
7.21
Pyridinic
Fe-Nₓ
40.76
8.21
13.25
15.35
43.12
24.57
7.00
15.16
44.97
25.46
7.21
Pyrrolic
Graphitic
N-Oₓ
37.28
5.81
33.45
13.54
7.95
13.79
The apparent surface area of the synthesized materials was estimated with BET from the isotherm
shown in figure S4. A similar trend to that reported in previous work by Ferrandon et al. 30 and Wu
et al31 is observed in 2a. The apparent surface area increases with the synthesis temperature
6
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901196
ChemCatChem
between 750° to 900°C from 536 to 688 m2/g and decreases after 900°C. In our case, however, the
FeNC 1050 shows a larger area which could be a result of the change in the carbon structure
observed in TEM.
The micropore area, shown in figure 2b, however exhibits a different behavior. As a general trend,
higher temperatures during the synthesis resulted in a loss in micropore area being the catalyst
treated a 750°C the one with the highest micropore area. For the ORR the micropore area has been
shown to be more important than the specific area in determining the activity the MNC catalysts.
35,36 In general, large micropore area correlates with a large activity towards the ORR, which can be
attributed to higher gas uptake in the microporous structure. If a similar trend is followed during
the CO2RR, we would expect a higher activity of the FeNC catalyst.
Finally, we also investigated the relative electrochemical surface area (ECSA) from double layer
capacity measurements (Figure 2c and 2d). The larger capacitive current observed on the catalyst
prepared at 750°C suggests that this material possess the highest electrocatalytic surface area. Similar
to the results from the micropore area there is a decrease in ECSA with increasing the synthesis
temperature. Nevertheless, the difference between the catalyst prepare at the 4 higher temperatures is
not significant, suggesting that the difference in catalytic performance between these materials can
mostly be attributed to differences in the nature of the active sites.
7
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901196
ChemCatChem
Figure 2: a) Apparent surface area estimated as a function of heat treatment temperature by BET b)
Micropore surface area as a function of heat treatment temperature c) Comparison of capacitive
current as an estimation of electrochemical surface area of the different catalysts. (CVs recorded at
1 mV/s). d) Double-layer capacitance current on the FeNC catalysts estimate by cyclicvoltammetry
as a function of scan rate
Electrocatalytic testing was first carried out by cyclic voltammetry (CV) in presence and absence of
CO2 in a 0.1 M phosphate buffer (Figure 3). The Faradaic currents observed in CO2‐free electrolyte
are entirely attributed to the HER, while the Faradaic currents in CO2‐saturated phosphate buffer
include the contribution of two processes the HER and the CO2RR. The observed currents on the
FeNC catalysts are clearly higher than on the metal free catalyst (Figure S6) confirming the important
role that the metal center has on the catalytic process. The incorporation of Fe to the NC material
results in a current increase of about 6 times, which indicates that most of the current during the
CO2RR can indeed be attributed to the presence of iron. The current from the metal free catalyst,
however, is also larger than that of the clean GC plate, showing that despite their lower activity the
N functionalities are also involved in the reduction process.
We notice that all 5 catalysts are active towards the competing process as there is a clear reduction
current in the absence of CO2. However, in all cases there is a larger current when CO2 is bubbled
8
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901196
ChemCatChem
through the electrolyte, which indicates that CO2 is also being reduced when applying negative
potentials.
Figure 3: Evaluation of activity for CO2RR vs HER of each FeNC catalyst treated at a) 750, b) 825, c)
900, d) 975 and e) 1050 °C. CVs were acquired at a scan rate of 1 mV/s using a KH2PO4/K2HPO4 ( 0.1
M) electrolyte with a catalysts loading of 0.75mg/cm2
In figure 3 we observed that the FeNC 750 is the most active catalyst both in the presence and
absence of CO2. This is consistent with the larger electrochemical area indicated by the capacity
measurements as well as the larger micropore area. This suggests that higher activity of FeNC 750
can be attributed to a difference in microstructure. The influence of different possible active sites,
however, should also be consider, as more nitrogen has been incorporated to the carbon structure
of this catalyst. The catalytic activity of the remaining 4 materials is lower than of FeNC 750,
however, no significant difference between them is observed, indicating that the activity of these
materials indeed correlates with the relative ECSA.
When evaluating CO2RR catalysts, in addition to the activity, a key parameter is the selectivity of the
process. A first indication of it is given by the difference in onset potential for the CO2RR and HER.
(Figure S7) For our family of catalysts, it seems that this difference is the smallest on FeNC 750
suggesting that despite the high activity this is not a particularly selective catalyst and that most of
the current can be attributed to the HER. By contrast, the difference in onset potential for FeNC
9
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901196
ChemCatChem
1050 is clearly the largest which is an indication that this is the most selective material for the
CO2RR.
To corroborate that these materials are in fact reducing CO2 into carbon-based products, we
performed CO2 electrolysis at a constant potential. The gas phase was analyzed via gas
chromatography allowing to quantify the gas products. Consistent with previous studies, CO and H2
were identified as the major reaction products, together with trace amounts of CH4.
The obtained faradaic efficiency on the 5 tested materials at -0.95 V vs NHE is shown in figure 4a.
Herein, it is clear that the selectivity towards the CO2RR is enhanced by preparing the FeNC catalysts
at higher heat treatment temperatures. As suggested by the cyclic voltammetry (Figure 3), FeNC
1050 had the highest faradaic efficiency towards both CO and CH4 production reaching 62.7% and
2.0% respectively at -0.95V vs NHE. A similar trend is observed in the potential range between -
0.85V and -1.05V vs NHE as shown in figure 4b and 4c confirming that increasing the synthesis
temperature of polyaniline derived FeNC catalysts has a beneficial effect on the selectivity towards
the CO2RR. Interestingly, the metal free catalyst also exhibited the ability to reduce CO2 into CO,
(Figure S8). The faradaic efficiency, however, was only 13.1% after 15 minutes and 11.6% after 60
minutes of electrolysis and no CH4 formation was observed. This value is clearly lower compared to
the metal containing materials, showing that despite some activity of the N functionalities they are
not selective towards the CO2RR.
Figure 4: Selectivity after 15 minutes of CO2 electrolysis at a constant potential. a) Comparison of
product distribution at a constant potential of -0.95V vs NHE b) CO faradaic efficiency as a function
of potential and c) CH4 faradaic efficiency as a function of potential. Electrolyte KH2PO4/K2HPO4 (0.1
M)
We note that the CO selectivity is lower than that reported on some other FeNC catalysts, which can
partially be attributed to a local pH effect. Most of the CO2RR studies are carried out in 0.1M KHCO3
10
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901196
ChemCatChem
electrolyte which has a lower buffer capacity resulting in a higher local pH in comparison with
KH2PO4/K2HPO4 buffer. Working in close to neutral pH at the interphase is known to favor the
24
production of CH4 and H2 lowering the CO selectivity. Our results, however, are comparable with
those obtained in KH2PO4/K2HPO4 buffer.24 By contrast, the selectivity towards CH4 is higher than
that reported on other MNC catalysts (Table S1). As we discussed previously, the close to neutral
local pH favors the production of CH4, however, the 2.0% faradaic efficiency is higher than that
reported on FeNC in the same electrolyte. This difference with previous results indicates that the
material structure is also involved in the observed high CH4 selectivity.
To understand this difference in electrocatalytic behavior, it is important to look into the catalysts
structure. As we have previously discussed, the ECSA has an effect on the catalyst activity. To
understand the differences in selectivity, we turn to the different N functionalities. There is a clear
difference in the concentration of pyridinic N, which has been proposed as possible active site for
CO2RR.26,27 In our study however, a lower concentration of pyridinic N correlates with a higher
selectivity towards the CO2RR. We note that, different studies on N doped carbon have reported
the production of both CO and H2. Our results show that under the same conditions the metal free
NC catalyst yields a high selectivity toward the competing process of the HER. This shows that,
despite some activity of the N functionalities towards the CO2RR, they are more active towards HER.
The high selectivity of the NC catalyst towards hydrogen production and its low catalytic activity,
are strong indicators that the predominant active sites involved are metal sites, most likely iron
coordinated to nitrogen to form FeNx.
Table 1 shows that a higher heat treatment temperature also favors the formation of such FeNx
sites. MNx functionalities have been proposed as highly selective sites towards the CO2RR. This has
been attributed to the nature of the isolated active sites which hinders the HER reaction due to a
low binding energy of *H at the on top site. This low binding energy to *H gives an advantage to
-
FeNx moieties as highly selective sites for the CO2RR31 on which the key intermediate (FeNC)-CO2
is formed without competing with (FeNC)-H formation. Consistently, experimental studies on NiNC
catalysts containing exclusively NiN4 reported faradaic efficiency up to 99% towards CO.32 Therefore,
the difference in catalytic activity of our tested materials can be attributed mainly to the
concentration of FeNx sites, which are not only the predominant active sites on MNC catalysts, but
they are also highly selective in reducing CO2 to CO. In addition, the higher synthesis temperature
increases the graphitization of the FeNC materials which also lowers the concentration of pyridinc
11
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901196
ChemCatChem
N which, we hypothesizes, are contributing to the competitive process of HER, despite some activity
towards the CO2RR.
More interestingly, this study shows that the temperature of the heat treatment has a clear effect
on polyaniline derived FeNC materials. In particular, we observed that the concentration of FeNX
functionalities, with respect to other N functionalities, increases with the temperature of the heat
treatment during the catalysts synthesis. As a result, the selectivity of the process is clearly
enhanced despite some losses in activity which we attribute to a change in microstructure of the
carbon support reducing the electrocatalytic surface area.
Finally, we have carried out stability measurements by performing 16 hours electrolysis at a constant
potential of -0.95V vs NHE, using three of our catalysts (FeNC 750, FeNC 900 and FeNC 1050). As it
can be seen in figure 5a, the three catalysts present a similar behavior, showing a reduction of
current density which indicates a loss in activity. Furthermore, figure 5b shows that the CO Faradaic
Efficiency also decreases, especially during the first three hours of reaction. We attributed this loss
in activity and selectivity to the blocking of the active site by *CO or other reaction intermediates.
FeNx functionalities are known to bind strongly to *CO and other intermediates bound through the
C atom. By contrast, the also active porphyrinic N should not be blocked and their contribution
becomes more important resulting in a higher faradaic efficiency towards H2. Interestingly, the
selectivity towards CH4 also increased during the first three hours of reaction reaching a constant
value of 2.8%, which can mainly by attributed to the lower overall currents. These long electrolysis
experiments exhibit another important challenge when it comes to CO2RR catalysts: the stability. In
this case the metal center is believed to be key in the observed deactivation. Working with metals
that bind weakly to CO should be beneficial for long term electrolysis, despite not producing any
products “byond” CO.
12
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901196
ChemCatChem
Figure 5: Stability test for 16 hours of electrolysis at -0.95 V vs NHE in KH2PO4/K2HPO4 (0.1 M) for
FeNC 750 (black) FeNC 900 (red) and FeNC 1050 (blue) a) Recorded current density b)CO Faradaic
efficiency c) CH4 faradaic efficiency
Conclusions
For this work, we have prepared 5 polyaniline-derived catalysts and shown that by simply changing
the synthesis temperature we dramatically affect the catalytic performance of this class of
materials. In particular, we observed that the catalyst treated at the lowest temperature (FeNC750)
has the largest electrochemical surface area and micropore area, resulting in a higher activity. The
selectivity towards the CO2RR, however, is clearly enhanced when the heat treatment temperature
is increased. The improvement in the catalytic performance of the FeNC catalyst prepared at higher
temperatures is associated with a larger concentration of FeNx which have been proposed to be
highly selective sites for the CO2RR. These results highlight the important role played by FeNx sites
during the CO2 electrochemical reduction on MNC catalysts. Furthermore, we show that the
concentration of such sites can be tuned by changing the synthesis parameters.
Experimental Section
Material synthesis
For the FeNCs synthesis, we first activated approximately 300 mg of Black Ketjen Carbon EC600
(Akzo Nobel) by heating it under reflux in 35% HNO3 solution. After 5 hours, the mixture was let to
cool down and diluted with ≈500 mL of water, before filtering. Afterwards the FeNC materials were
prepared by dissolving 1 mL of aniline 99.0% in 100 mL of an HCl 0.5 mol/L solution. To this mixture,
we added 2.5 g of FeCl3⋅6H2O 99% (our metal source) with constant stirring. Once the solution was
homogeneous, we added a (NH4)2S2O8 solution (2.5 g of (NH4)2S2O8 in 150 mL HCl 0.5 mol/L) by drop
maintaining a 0 °C temperature during the addition to promote a slow aniline polymerization. After
5 h of stirring, 0.2 g of the previously activated Black Ketjen Carbon were added the mixture and left
for 48 hours. Then the water was evaporated and the recovered powder was grounded in an agate
mortar and heated by 1 h under nitrogen atmosphere at different temperatures for each catalyst
(750, 825, 900, 975 and 1050 °C). Subsequently, the solid was dispersed in H2SO4 2 mol/L solution
and refluxed for 3 h. The solid was recovered by vacuum filtration, grounded and heated again at
the same temperature of the first treatment. We finally gave a second H2SO4 reflux and a third
heating to each solid. The heat treatments were performed using a MTI oven (model OTF-1200X).
13
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901196
ChemCatChem
The iron free NC catalyst was prepared following the same procedure without adding FeCl3⋅6H2O
and with a pyrolysis temperature of 900°C
Material characterization
Powder X-ray diffraction (PXRD) patterns were obtained with Bruker D8 Advance diffractometer
equipped with a Cu-Kα source (λ=1.54056 Å). TEM images were acquired with a JEOL JEM-2010F
microscope applying an accelerating voltage of 200 kV. The textural properties of the samples were
obtained by nitrogen physisorption at 77 K using a Micromeritics TriStar surface area and porosity
analyzer. Before nitrogen adsorption, the samples were outgassed at 543 K for 12 h. The BET, t-plot
and BJH methods were used to calculate specific surface area, pore volume and pore size
distributions.
X-ray Photoelectron Spectroscopy measurements were performed using a JEOL JPS-9200, equipped
with a dual Mg/Al X-ray source, the Mg source was used with photon energy of 1253.6 eV at 200 W,
the analysis area was 3 mm2, and all samples were analyzed under high vacuum (1 × 10-8Torr). Charge
correction was made by means of the adventitious carbon (C1s) XPS signal at 284.5 eV. Additionally,
elemental analysis were carried out using a Thermo Scientific Flash 2000 elemental analyzer, and a
Mettler Toledo XP6 microbalance. Two verification standards were used for the elemental analysis:
Methionine (N=9.35 %, C=40.25 %, H=7.43 % and S=21.49 %) and Sulfanilamide (N=16.22 %, C=41.79
%, H=4.66 % and S=18.60 %), both reference material from Thermo Scientific.
Electrochemical measurements
Both capacitive current and activity measurements were controlled by a BioLogic SP-300
potentiostat and were done in a conventional three electrode cell. We used a Ag(s)|AgCl(s)|NaCl(ac)(3
mol/L) electrode (Als Co 012167 RE-1B) as a reference electrode and a platinum mesh as
counterelectrode. The working electrode was prepared by depositing different volumes of catalyst
suspension on 1 cm2 of glassy carbon (25 μL for activity measurements and 75 μL for capacitive
current experiments). This catalyst suspension was made sonicating a mixture of 15 mg of catalyst
with 50 μL of Nafion 117, 450 μL of isopropyl alcohol and 500 μL of ultrapure water by 1 min with a
probe sonicator. The electrodes were submerged in a H2KPO4/HK2PO4 buffer solution (0.1 M, pH=7).
Both the capacitive current and the materials activity were measured by cyclic voltammetry. The
CVs to evaluate the catalysts’ activity were acquired at 1 mV/s at potentials within -0.9 and -0.15 V
14
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901196
ChemCatChem
vs NHE (under nitrogen atmosphere for HER or with CO2 bubbling for CO2RR activity). On the other
hand, CVs used to measure the capacitive current were done at 1, 3, 5, 7, and 9 mV/s at potentials
between -0.53 and -0.15 V vs NHE under nitrogen atmosphere.
Electrolysis experiments
CO2 electrolysis was carried out in a custom-made two-compartment cell, in which the working
electrode was separated from the counter electrode by Nafion membrane. Resistance between
reference and working electrode was measured with Potential Electrochemical Impedance
Spectroscopy and 50% of it was corrected by the software.
After 15 minutes at constant potential, the gas products were analyzed with a gas chromatograph
(Shimadzu GC 2014) equipped with a thermal conductivity detector (TCD) and a flame ionization
detector (FID). The gaseous compounds H2, N2, O2, CH4 and CO were separated in a molecular sieve
column (Alltech, part no. 57732, 1.65 m × 1/8 in., molecular sieve 13X, 60/80 mesh).
Acknowledgements
The authors acknowledge funding given by PAPIIT, Project Number IA101017S. In addition, we thank
Dr. Gustavo López Téllez from CCIQS UAEM-UNAM, for the support with the XPS measurements, Dr.
Samuel Tehuacanero Cuapa from IF-UNAM for his help in the acquisition of the TEM images and Q. María
de la Paz Orta Pérez from IQ-UNAM for her support with the elemental analysis experiments. Finally we
also thank Prof. Peter Strasser for letting us carried out electro catalytic tests at his laboratory in the
Technical University of Berlin.
Keywords
CO2 electroreduction, electrocatalysis, Polyaniline-derived carbon materials, single-site solid catalysts
References
(1)
(2)
(3)
(4)
K. M. K. Yu, I. Curcic, J. Gabriel, Tsang, S. C. E. ChemSusChem. 2008, 1, 893-899.
M. Gattrell, N. Gupta, A. Co. Energy Convers. Manage. 2007, 48, 1255-1265.
Y. Hori, A. Murata, R. Takahashi. J. Chem. Soc., Faraday Trans. 1. 1989, 85, 2309-2326.
X. Wang, A. S. Varela, A. Bergmann, S. Kühl, P. Strasser. ChemSusChem. 2017, 10, 4642-
4649.
7059.
358.
(5)
(6)
(7)
K. P. Kuhl, E. R. Cave, D. N. Abram, T. F. Jaramillo. Energy Environ. Sci. 2012, 5, 7050-
R. Kas, R. Kortlever, H. Yılmaz, M. T. M. Koper, G. Mul. ChemElectroChem. 2015, 2, 354-
E. Bertheussen, T. V. Hogg, Y. Abghoui, A. K. Engstfeld, I. Chorkendorff, I. E. L. Stephens.
ACS Energy Lett. 2018, 3, 634-640.
15
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901196
ChemCatChem
(8)
(9)
H. Mistry, A. S. Varela, S. Kühl, P. Strasser, B. R. Cuenya. Nat. Rev. Mater. 2016, 1, 16009.
M. E. Dry. J. Chem. Technol. Biotechnol. 2002, 77, 43-50.
(10)
Q. Lu, J. Rosen, Y. Zhou, G. S. Hutchings, Y. C. Kimmel, J. G. Chen, F. A Jiao. Nat. Commun.
2014, 5.
(11)
S. Mezzavilla, S. Horch, I. E. L. Stephens, B. Seger, I. Chorkendorff. Angew. Chem. 2019,
131, 3814-3818.
(12)
(13)
Y. Chen, C. W. Li, M. W. Kanan. J. Am. Chem. Soc. 2012, 134, 19969-19972.
A. S. Varela, N. Ranjbar Sahraie, J. Steinberg, W. Ju, H.-S. Oh, P. Strasser. Angew. Chem.,
Int. Ed. 2015, 54, 10758-10762.
(14)
K. Jiang, S. Siahrostami, A.J. Akey, Y. Li, Z. Lu, J. Lattimer, Y. Hu, C. Stokes, M. Gangishetty,
G. Chen, Y. Zhou, W. Hill, W.-B. Cai, D. Bell, K. Chan, J. K. Nørskov, Y. Cui, H. Wang. Chem. 2017, 3, 1-11.
(15)
W. Ju, A. Bagger, G.-P. Hao, A. S. Varela, I. Sinev, V. Bon, B. Roldan Cuenya, S. Kaskel, J.
Rossmeisl, P. Strasser. Nat. Commun. 2017, 8, 944.
(16)
W. Bi, X. Li, R. You, M. Chen, R. Yuan, W. Huang, X. Wu, W. Chu, C. Wu, Y. Xie. Adv.
Mater. 2018, 30, 1706617.
(17)
232, 512-520.
(18)
A. Roy, D. Hursán, K. Artyushkova, P. Atanassov, C. Janáky, A. Serov. Appl. Catal. B. 2018,
X.-M. Hu, H. H. Hval, E. T. Bjerglund, K. J. Dalgaard, M. R. Madsen, M.-M. Pohl, E. Welter,
P. Lamagni, K. B. Buhl, M. Bremholm, M. Beller, S. U. Pedersen, T. Skrydstrup, K. Daasbjerg. ACS Catal.
2018, 8, 6255-6264.
(19)
N. D. Leonard, S. Wagner, F. Luo, J. Steinberg, W. Ju, N. Weidler, H. Wang, U. I. Kramm,
P. Strasser. ACS Catal. 2018, 8, 1640-1647.
(20)
(21)
(22)
D. Liu, L. Tao, D. Yan, Y. Zou, S. Wang. ChemElectroChem. 2018, 5, 1775-1785.
G. Wu, K. L. More, C. M. Johnston, P. Zelenay. Science. 2010, 332, 443-447.
S. Pylypenko, S. Mukherjee, T. S. Olson, P. Atanassov. Electrochim. Acta. 2008, 53, 7875-
7883.
(23)
V. Tripkovic, M. Vanin, M. Karamad, M. E. Björketun, K. W. Jacobsen, K. S. Thygesen, J.
Rossmeisl. J. Phys. Chem. C. 2013, 117, 9187-9195.
(24)
A. S. Varela, M. Kroschel, N. D. Leonard, W. Ju, J. Steinberg, A. Bagger, J. Rossmeisl, P.
Strasser. ACS Energy Lett. 2018, 812-817.
(25)
F. Pan, H. Zhang, K. Liu, D. Cullen, K. More, M. Wang, Z. Feng, G. Wang, G. Wu, Y. Li. ACS
Catal. 2018, 8, 3116-3122.
(26)
C. Yan, H. Li, Y. Ye, H. Wu, F. Cai, R. Si, J. Xiao, S. Miao, S. Xie, F. Yang, Y. Li, G. Wang, X.
Bao. Energy Environ. Sci. 2018, 11, 1204-1210.
(27)
N. Leonard, W. Ju, I. Sinev, J. Steinberg, F. Luo, A. S. Varela, B. Roldán, P. Strasser. Chem.
Sci. 2018, 9(22), 5064–5073.
(28)
P. P. Sharma, J. Wu, R. M. Yadav, M. Liu, C. J. Wright, C. S. Tiwary, B. I. Yakobson, J. Lou,
P. M. Ajayan, X.-D. Zhou. Angew. Chem., Int. Ed. 2015, 54, 13701-13705.
(29)
Vajtai, B. I. Yakobson, J. Lou, P. M. Ajayan. Nano Lett. 2016, 16, 466-470.
(30) M. Ferrandon, A. J. Kropf, D. J. Myers, K. Artyushkova, U. Kramm, P. Bogdanoff, G. Wu,
C. M. Johnston, P. Zelenay. J. Phys. Chem. C. 2012, 116, 16001-16013.
(31) G. Wu, C. M. Johnston, N. H. Mack, K. Artyushkova, M. Ferrandon, M. Nelson, J. S.
J. J. Wu, M. J. Liu, P. P. Sharma, R. M. Yadav, L. L. Ma, Y. C. Yang, X. L. Zou, X. D. Zhou, R.
Lezama-Pacheco, S. D. Conradson, K. L. More, D. J. Myers, P. Zelenay. J. Mater. Chem. 2011, 21, 11392-
11405.
(32)
T. Möller, W. Ju, A. Bagger, X. Wang, F. Luo, T. Ngo Thanh, A. S. Varela, J. Rossmeisl, P.
Strasser. Energy Environ. Sci. 2019, 12, 640-647.
(33)
(34)
A. Bagger, W. Ju, A. S. Varela, P. Strasser, J. Rossmeisl. Catal. Today. 2017, 288, 74-78.
X. Li, W. Bi, M. Chen, Y. Sun, H. Ju, W. Yan, J. Zhu, X. Wu, W. Chu, C. Wu, Y. Xie. J. Am.
Chem. Soc. 2017, 139, 14889-14892.
16
This article is protected by copyright. All rights reserved.

10.1002/cctc.201901196
ChemCatChem
(35)
(36)
F. Jaouen, S. Marcotte, J.-P. Dodelet, G. Lindbergh. J. Phys. Chem. B. 2003, 107, 1376-
1386.
F. Jaouen, M. Lefèvre, J.-P. Dodelet, M. Cai. J. Phys. Chem. B. 2006, 110, 5553-5558.
17
This article is protected by copyright. All rights reserved.