Selective ligand modification of cobalt porphyrins for carbon dioxide electrolysis: Generation of a renewable H2/CO feedstock for downstream catalytic hydrogenation

Article abstract of DOI:10.1016/j.ica.2020.119594

Catalytic hydrogenation is an attractive approach to produce green fuels and chemicals. The building blocks for these processes may be effectively produced from renewable power via direct electrochemical reduction of carbon dioxide in an aqueous media. For the first time, the impact of increasing the local proton concentration of cobalt porphyrin was examined by synthesizing new cobalt porphyrins 2, Co(o-OCH₃)TPP and cobalt porphyrin 3, Co(o-OH)TPP. Cobalt porphyrins coated on carbon paper converted carbon dioxide and water into a mixture of hydrogen and carbon monoxide in an aqueous electrolyte at near neutral pH. Increasing the local proton availability of the commercial cobalt porphyrin 1, accelerates hydrogen generation under heterogeneous conditions across the range of potentials tested (?0.85 to ?1.5 V vs. Ag/AgCl) and demonstrates high Faradaic efficiencies (ca. 90%) at low over-potentials (ca. 540 mV). The culmination of this work can help identify key parameters that facilitate generation of sustainable reagents for catalytic hydrogenation under practical and scalable conditions.

Full text of DOI:10.1016/j.ica.2020.119594

InorganicaChimicaActa507(2020)119594
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Selective ligand modification of cobalt porphyrins for carbon dioxide
electrolysis: Generation of a renewable H2/CO feedstock for downstream
catalytic hydrogenation
Joshua Jacka,b,d,1, Eunsol Parkc,1, Pin-Ching Manessb, Shaofeng Huangc, Wei Zhangc,⁎
,
⁎
Zhiyong Jason Renb,d,
a Department of Civil, Architectural and Environmental Engineering, University of Colorado Boulder, 4001 Discovery Drive, Boulder, CO 80301, United States
b Biosciences Center, Department of Energy-NREL, FTLB, 16253 Denver West Blvd, Golden, CO 80401, United States
c Department of Chemistry, University of Colorado Boulder, 215 UCB, Boulder, CO 80309, United States
d Department of Civil and Environmental Engineering, Princeton University, 213 Andlinger Center, Princeton, NJ 08540, United States
A R T I C L E I N F O
A B S T R A C T
Keywords:
Catalytic hydrogenation is an attractive approach to produce green fuels and chemicals. The building blocks for
these processes may be effectively produced from renewable power via direct electrochemical reduction of
carbon dioxide in an aqueous media. For the first time, the impact of increasing the local proton concentration of
cobalt porphyrin was examined by synthesizing new cobalt porphyrins 2, Co(o-OCH3)TPP and cobalt porphyrin
3, Co(o-OH)TPP. Cobalt porphyrins coated on carbon paper converted carbon dioxide and water into a mixture
of hydrogen and carbon monoxide in an aqueous electrolyte at near neutral pH. Increasing the local proton
availability of the commercial cobalt porphyrin 1, accelerates hydrogen generation under heterogeneous con-
ditions across the range of potentials tested (−0.85 to −1.5 V vs. Ag/AgCl) and demonstrates high Faradaic
efficiencies (ca. 90%) at low over-potentials (ca. 540 mV). The culmination of this work can help identify key
parameters that facilitate generation of sustainable reagents for catalytic hydrogenation under practical and
scalable conditions.
Electrocatalytic CO2 reduction
Heterogeneous electrocatalysis
Local proton source
Modified ligand porphyrin
Catalytic hydrogenation
1. Introduction
hydrogenation processes that use H2 and CO as the feedstock to build
larger molecules [8,9]. These catalytic processes can be used to react H2
Selective and efficient conversion of carbon dioxide (CO2) to high
energy density fuels is paramount for a closed-carbon energy future.
Electrochemical CO2 reduction is an attractive platform for CO2 re-
duction as it can be directly integrated with renewable energy sources.
The injection of an electron into CO2, however, requires highly negative
potentials (large energy investment) and the kinetics are quite sluggish
under standard conditions [1–3]. On-going investigations have thus
been largely focused on the development of synthetic catalysts, re-
garding the formation of reduction products requiring four or more
electrons (i.e. methane, ethanol, oxalate) [4–6]. After several years, this
work has resulted in limited success as it has been faced with tre-
mendous inherent challenges involving high over-potentials and com-
plex parallel reactions schemes that hinder overall product efficiencies
[7]. In light of these findings, some have shifted focused away from
direct electrochemical CO2 reduction and back to catalytic
and CO with CO2 to generate methane, methanol, formaldehyde, or
higher alcohols [10]. In general, hydrogenation processes are far more
mature than their electrochemical counterparts and can support ex-
cellent energetic efficiencies for carbon intensive molecules [11]. For
example, Wang et al. recently reported a very high selectivity for
ethanol (30.1%) via CO catalytic hydrogenation using carbon nanotube
supported cobalt-copper with a H2/CO feedstock of 2:1 [12]. Tradi-
tionally, the sources of H2/CO have come from the deconstruction of
agricultural feedstock that have large water, land, and nutrient re-
quirements as well as a fundamentally low energy to product conver-
sion efficiency [10]. Alternatively, catalytic hydrogenation could be
directly integrated with renewable energy sources by identifying a
versatile catalyst that could electrochemically produce a range of H2/
CO blends (from a waste CO2 feedstock and water) that accommodate a
variety of downstream hydrogenation processes. In the early work on
⁎ Corresponding authors.
E-mail addresses: Wei.Zhang@colorado.edu (W. Zhang), zjren@princeton.edu (Z.J. Ren).
1 These authors have contributed equally.
https://doi.org/10.1016/j.ica.2020.119594
Received 19 December 2019; Received in revised form 12 March 2020; Accepted 12 March 2020
Availableonline13March2020
0020-1693/©2020ElsevierB.V.Allrightsreserved.

J. Jack, et al.
InorganicaChimicaActa507(2020)119594
electrochemical CO2 reduction, several catalysts with this capability
were identified. However, these catalysts were generally not further
optimized, in terms of energy efficiency or stability, as H2 or CO (alone)
are relatively low value products. As such, most of these catalysts were
discarded and research efforts progressed towards the formation of
hydrocarbons. In lieu of this methodology, Hu et al. recently reported
that cobalt tetraphenyl porphyrins could produce mixtures of H2/CO
with extremely high efficiencies under heterogeneous conditions [13].
Early work with analogous metalloporphyrins showed that the presence
of Lewis acids such as Mg2+ or Ca2+ could enhance the catalytic CO2
reduction efficiency and stability of molecules [14,15]. These studies
proposed a push–pull mechanism where the electro-reduced metal
species pushes an electron pair to the CO2 molecule and the electron-
deficient acid synergist promotes the separation of one of the C–O
bonds [16,17]. Following these observations some have demonstrated
the organic framework of porphyrins modified by electron withdrawing
groups can benefit the electronic structure towards improved CO2 re-
duction capabilities [18]. In line with these findings, Costentin et al.
found that the electroreduction of CO2 to CO of an iron-based porphyrin
monomer could be greatly enhanced by the addition of hydroxyl groups
in all phenyl group ortho positions under homogenous conditions [19].
This and subsequent reports concluded that the local addition of acids
assisted in the stability of a key intermediate, [Metal-(P)−(CO2]−, in
the formation of H2/CO and therefore improved overall Faradaic effi-
ciencies of the process [19,20]. In the present study, we applied this
theory and synthesized new cobalt based catalysts, cobalt porphyrin 2,
Co(o-OCH3)TPP and cobalt porphyrin 3, Co(o-OH)TPP to study the
impact of increasing the local proton source on H2/CO efficiencies
under heterogeneous conditions. It is important to note that past studies
primarily characterized catalysts under only homogenous conditions,
that are not practical for scale-up. The results presented, hereto, de-
monstrate high yields and efficiencies for a variety of H2/CO blends
using the novel porphyrin catalysts in aqueous conditions. Moreover,
this work provides valuable experimental insight on the role of local
proton concentration in CO2 electro-reduction and demonstrates a
variety of operating conditions to produce tunable H2/CO mixtures that
can be coupled to downstream hydrogenation processes towards the
generation of green fuels and chemicals.
2.2. Cyclic voltammetry
A platinum wire (BASi-MF-1033) was employed as counter elec-
trode along with an Ag/AgCl reference electrode and the appropriate
working electrode (GC disk or carbon paper). 2 mM of catalyst and
0.1 M tetrabutylammonium hexafluorophosphate (NBu4PF6) as the
supporting electrolyte were added. Prior to analysis, the electrolyte
solutions were saturated with high purity Argon or CO2 gas.
2.3. Gas analysis
Gas samples were obtained directly from the headspace of the
electrochemical cell using a 250 μl gastight syringe (Hamilton-1700
series) and were immediately analyzed on an Agilent Technologies
7890A gas chromatography system equipped with a thermal con-
ductivity detector (TCD) and packed column [22,23].
2.4. Faradaic efficiency calculation
The Faradaic efficiency (FE) of the CO2 electrochemical reduction
products was calculated as
Z × n × F
FE =
Q
where n is the number of the electrons needed for CO2 reduction or HER
(2 for both CO and H2), Z is the moles of products, F is the Faraday
constant, and Q is charged transferred during electrolysis.
3. Results and discussion
To examine the impact of local proton availability on CO2 reduc-
tion, we have synthesized two modified cobalt porphyrins 2 and 3 by
the addition of phenolic methoxyl or hydroxyl groups in all phenyl
group ortho positions (Fig. 1). Their catalytic activity was then com-
pared to the performance of the commercial porphyrin 1. Substitution
of hydrogen with methoxy and hydroxyl groups increased the local
proton availability. Prior to cobalt-metalation, the structure of the two
free base porphyrins, H2(o-OCH3)TPP and H2(o-OH)TPP, were con-
firmed by nuclear magnetic resonance spectroscopy (NMR) in d-CHCl3
and d-CH3OH, respectively (Figs. S1, S2). The as-prepared cobalt por-
phyrins 2 and 3 were then finished via a final metalation step and
characterized by ultraviolet–visible (UV–Vis), Fourier transform-in-
frared spectroscopy (FT-IR), electrospray ionization mass spectrometer
(ESI-MS), thermogravimetric analysis (TGA), and CHN elemental ana-
lysis (Figs. S3–S5, Table S1, S2). The analyses confirmed that the
2. Experimental
Cobalt(II)-5,10,15,20-tetrakis(2′,6′-dimethoxyphenyl)-
21H,23H-porphyrin,Co(o-OCH3)TPP:
Solid
H2(o-OCH3)TPP
(0.1453 g, 0.170 mmol) and anhydrous CoCl2 (II) (0.441 g, 3.40 mmol)
were dissolved in anhydrous tetrahydrofuran (THF, 30 mL) under inert
N2 gas and 2,6-lutidine (0.060 mL, 0.51 mmol) was added. The solution
was then refluxed for two hours. After removing THF solution, the
mixture was dissolved in dichloromethane (DCM) and washed with
deionized water three times [19,21].
Cobalt(II)-5,10,15,20-tetrakis(2′,6′-dihydroxyphenyl)-
21H,23H-porphyrin, Co(o-OH)TPP: Co(o-OH)TPP was prepared in
the same fashion except ethyl acetate was used to dissolve the mixture
after removing the THF solution.
2.1. Material characterization
1H NMR spectra were recorded on a Bruker Avance-III 300 NMR
spectrometer. Fourier transform infrared (FT-IR) spectra were mea-
sured with an Agilent Technologies, Cary 630. UV–Vis spectra were
recorded on an Agilent 8453. ESI-MS measurements were performed on
a Synapt G2 HDMS. Elemental analysis of as-obtained compounds was
conducted on a CHN analyzer from Exeter analytical (model CE440
CHN).
Fig. 1. The chemical structure of cobalt porphyrins with modified ligands (2
and 3) and commercial one (1).
2

J. Jack, et al.
InorganicaChimicaActa507(2020)119594
Fig. 2. CO2 reduction capabilities of (a) commercial porphyrin 1; (b) as-obtained cobalt porphyrin 2; (c) as-obtained cobalt porphyrin 3; (d) catalysts comparison.
Vertical dashed lines depict shift of onset potentials from aprotic CO2 conditions (light green) to acid conditions (dark blue). Conditions: Ag/AgCl reference electrode,
glass carbon working electrode, Pt anode for counter electrode, 1 mM catalyst, and 0.1 M NBu4PF6 dissolved in DMF.
metalation step was effective and the modified cobalt porphyrin 2 and 3
were successfully synthesized. FT-IR spectra of cobalt 2 showed the
disappearance of the N–H peak (3300 cm) of H2(o-OCH3)TPP and
UV–Vis spectra of H2(o-OH)TPP showed a red shift after metalation.
ESI-MS of both as-obtained compounds revealed that the percent error
of experimental values was less than 2 ppm from the theoretical, further
confirming the structure of these compounds. CHN elemental analysis
provided the purity of the as-obtained cobalt porphyrin 2 and 3 and
TGA confirmed the presence of several equivalents of solvents mole-
cules, for example, tetrahydrofuran or water.
evolution reaction (HER) or a mixture of both. Consequently, controlled
potential electrolysis experiments were required to investigate the
product selectivity of each catalyst and elucidate the affinity for each
possible reaction mechanism.
To maintain the practicality of these findings, electrolysis was
conducted under heterogeneous conditions with an aqueous electrolyte
(at room temperature). Preliminary CVs of each of the catalysts (in
aqueous conditions) demonstrated that the onset potentials started at
slightly more positive potentials than −0.8 V vs. Ag/AgCl so electro-
lysis were carried out at moderate potentials in the range of −0.85 V to
−1.5 V vs. Ag/AgCl, in order to produce appreciable gaseous products
(Fig. S6). At each operating voltage, it was apparent that the as-ob-
tained cobalt porphyrins 2 and 3 favored the HER more than cobalt
porphyrin 1 under the same conditions. For instance, the Faradaic ef-
ficiencies (FEs) of cobalt porphyrin 1 for HER was only about a tenth of
that of cobalt porphyrin 2 and 3 after an hour electrolysis at −1.2 V vs.
Ag/AgCl (Fig. 3). This finding is significant as in comparable studies
with iron porphyrins (under homogenous conditions) enhanced cata-
lytic waves were attributed to only CO2 reduction [19]. Under hetero-
geneous conditions, the selectivity towards CO2 reduction via the
electron push-pull mechanism may have been lessened due to direct
competition of the attached acid synergist with the other acids in so-
lution (i.e. CO2 species and H2O). Using highly regarded DFT models
[24], it was anticipated that the increased local proton availability
would accelerate the stabilization of the cobalt-CO2 adduct via Eq. (4)
as part of the proton-concerted reaction pathway of CO2 towards CO.
Cyclic voltammetry (CV) of cobalt porphyrins 1–3 were obtained in
a DMF solution containing 0.1 M NBu4PF6 saturated with CO2 gas to
assess the fundamental impact of the local proton source on electro-
catalytic CO2 reduction (Fig. 2). Notably, an assortment of responses in
catalytic behavior were observed between the catalysts upon introdu-
cing varying quantities of protons (2 M, 10 M H2O) to the initially
aprotic solvent. Commercial cobalt porphyrin 1 seemed to be un-
affected by the addition of water with a Co2+–Co+1 activation peak
around −0.80 V vs. Ag/AgCl and the start of a major reduction event s
around −1.85 V vs. Ag/AgCl across each scenario. These findings are
similar to those of Hu et al. when introducing protons via phenol to
CoTPP immobilized on GC disk electrodes in DMF [13]. In contrast, the
as-obtained cobalt porphyrins 2 and 3 seemed to show enhanced cat-
alytic activity upon the addition of water. For example, the onset po-
tentials (i.e. potential at which sharp increase in reduction current was
observed) of both cobalt porphyrin 2 and 3 seemed to shift by ca.
300 mV to more positive potentials upon adding 10 M H2O. Re-
markably, the reduction current of cobalt porphyrin 3 was much higher
than others upon the addition of 10 M H2O reaching about twice that of
cobalt porphyrin 1 and 2 at −2.5 V vs. Ag/AgCl. These findings were
supported by similar trends of the Tafel plot constants as the as-ob-
tained catalysts achieved much smaller values in the presence of H2O
owing to their enhanced catalytic activities (Table S4). The culmination
of these results are in-line with the findings of others using modified
FeTPP catalysts for CO2 reduction under homogenous conditions [19].
It is important to note that at the potentials near where the activation
peaks start, the current can be attributed to CO2 reduction, hydrogen
[CoP]0 + e− → [CoP]−
(1)
(2)
(3)
(4)
[CoP]− + CO2 → [Co(P)−(CO2)]−
[CoP]0 + CO2 + H+ + e− → [CoP − COOH]0
[Co(P)−(CO2)]− + H+ → [Co(P)−(COOH)]0
As the equilibrium potential for Eq. (4) is small (−0.37 mV vs. Ag/
AgCl), it is likely that this reaction is spontaneous and non-limiting in
the potential range tested, thus the impact of increased proton avail-
ability was not observed [24]. The local proton source could have been
3

J. Jack, et al.
InorganicaChimicaActa507(2020)119594
Fig. 3. Cobalt porphyrin heterogeneous preparative scale electrolysis. Faradaic efficiencies of each catalyst at (a) −0.85 V vs. Ag/AgCl; (b) −1.2 V vs. Ag/AgCl; (c)
−1.5 V vs. Ag/AgCl. Hydrogen evolution reaction (HER) in dark blue and CO2 reduction reaction (CO2-RR) in light blue. (conditions: Ag/AgCl reference electrode,
catalyst deposited on carbon paper working electrode, graphite anode working electrode, 0.5 M KHCO3 electrolyte). (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
used for Eq. (3) but it seems that it was outcompeted by the initial HER
Eq. (5) or Eq. (6) (below) [19,24].
sensitive in near neutral pH conditions. The culmination of these
findings can help pinpoint crucial operational parameters that produce
tunable H2/CO mixtures, which can be used as sustainable building
blocks for green chemical synthesis in a coupled electrochemical-cata-
lytic hydrogenation reaction. Further optimization of the catalysts as
well as integration of hydrogenation processes towards hydrocarbon
production is currently underway.
[CoP]0 + H+ + e− → [Co(P)−(H)]0
[CoP]− + H+ + e− → [Co(P)−(H)]−
(5)
(6)
Minor products such as formic acid may have also been produced
but were likely limited by the high over-potential needed to drive the
initial reduction reactions (Eq. A.7, A.8) [24].
CRediT authorship contribution statement
Interestingly, heterogeneous testing conditions allowed for CO2 re-
duction to occur at very low over-potentials for cobalt porphyrin 3 (ca.
540 mV vs. Ag/AgCl). Furthermore, at potentials close to the onset (ca.
−0.8 V vs. Ag/AgCl), H2 pathways were limited and allowed for ex-
cellent CO2 reduction efficiencies. For instance, the CO2-RR FE of cobalt
porphyrin 3 was improved by ca. 20% when the operating potential
was increased from −1.5 V to −0.85 V vs. Ag/AgCl (Fig. 3). Similar
results for the commercial porphyrin 1 were also observed and have
been previously reported by others under heterogeneous conditions
[13,25,26]. Notably, the CO2 reduction capabilities of the commercial
cobalt porphyrin 1 were not improved by addition of local proton
sources. This suggests that improving proton-concerted pathways may
not be effective in enhancing CO2 reduction on cobalt porphyrins under
aqueous conditions. Nevertheless, overall efficiencies for each of the
catalysts for H2/CO products were excellent (typically ca. 90%) and
unique gas blends for each of the catalysts were observed. This holds
great promise for the use of these compounds to generate tunable H2/
CO feedstock for hydrogenation processes (Fig. S8).
Joshua Jack: Investigation, Formal analysis, Writing - original
draft, Writing - review & editing. Eunsol Park: Investigation, Formal
analysis, Writing - original draft, Writing - review & editing. Pin-Ching
Maness: Investigation. Shaofeng Huang: Investigation. Wei Zhang:
Conceptualization, Formal analysis, Supervision, Writing - original
draft, Writing
-
review
&
editing. Zhiyong Jason Ren:
Conceptualization, Formal analysis, Supervision, Writing - original
draft, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgement
4. Conclusion
This work was supported by University of Colorado Boulder and K.
C. Wong Education Foundation.
Cobalt porphyrins modified with hydroxy groups can produce CO
and H2 in an aqueous electrolyte at near neutral pH and at a low over-
potential of ca. 540 mV in heterogeneous conditions. Using newly
synthesized cobalt porphyrin 2 and 3 catalysts, the impact of increased
local proton availability was examined. This led to accelerated H2
production and suggested that CO production pathways are not proton
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2020.119594.
4

J. Jack, et al.
InorganicaChimicaActa507(2020)119594
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