Bio-inspired design: Bulk iron-nickel sulfide allows for efficient solvent-dependent CO2 reduction

Article abstract of DOI:10.1039/c8sc03555e

The electrocatalytic reduction of carbon dioxide (CO₂RR) to valuable bulk chemicals is set to become a vital factor in the prevention of environmental pollution and the selective storage of sustainable energy. Inspired by structural analogues to the active site of the enzyme CODH_Ni, we envisioned that bulk Fe/Ni sulfides would enable the efficient reduction of CO₂. By careful adjustment of the process conditions, we demonstrate that pentlandite (Fe_4.5Ni_4.5S₈) electrodes, in addition to HER, also support the CO₂RR reaching a peak faradaic efficiency of 87% and 13% for the formation of CO and methane, respectively at 3 mA cm^-2. The choice of solvent, the presence of water/protons and CO₂ solubility are identified as key-properties to adjust the balance between HER and CO₂RR in favour of the latter. Such experiments can thus serve as model reactions to elucidate a potential catalyst within gas diffusion electrodes.

Full text of DOI:10.1039/c8sc03555e

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Bio-inspired design: bulk iron–nickel sulfide allows
for efficient solvent-dependent CO2 reduction†
Cite this: DOI: 10.1039/c8sc03555e
ab
b
Stefan Piontek,a Kai junge Puring,
Daniel Siegmund,
Mathias Smialkowski,a
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Ilya Sinev,c David Tetzlaff,a Beatriz Roldan Cuenya
and Ulf-Peter Apfel
d
ab
*
The electrocatalytic reduction of carbon dioxide (CO2RR) to valuable bulk chemicals is set to become a vital
factor in the prevention of environmental pollution and the selective storage of sustainable energy. Inspired
by structural analogues to the active site of the enzyme CODHNi, we envisioned that bulk Fe/Ni sulfides
would enable the efficient reduction of CO2. By careful adjustment of the process conditions, we
demonstrate that pentlandite (Fe4.5Ni4.5S8) electrodes, in addition to HER, also support the CO2RR
reaching a peak faradaic efficiency of 87% and 13% for the formation of CO and methane, respectively at
3 mA cmꢀ2. The choice of solvent, the presence of water/protons and CO2 solubility are identified as
key-properties to adjust the balance between HER and CO2RR in favour of the latter. Such experiments
can thus serve as model reactions to elucidate a potential catalyst within gas diffusion electrodes.
Received 10th August 2018
Accepted 5th November 2018
DOI: 10.1039/c8sc03555e
rsc.li/chemical-science
HER or CO2RR activity usually depends on the process condi-
tions applied. E.g. Cu and pyrite (FeS2) were both reported to
Introduction
facilitate H2 generation as well as CO2 reduction at various
conditions and further support the Janus-faced character of
both processes.13–17
Contrary to articial catalysts,18–20 nature has developed
sophisticated machineries to allow for an efficient CO2 reduc-
tion. E.g. the carbon monoxide dehydrogenase21 allows for
selective reduction of CO2 to CO at ꢀ0.52 V vs. NHE and
ambient pressure with turn over frequencies of up to 700
molecules per h.22–24
The efficient enzymatic reactivity is related to its unique [Ni–
4Fe–5S] cluster (Fig. 1a), a nely enzyme regulated amount of
protons/water at the reactive center, as well as a directed CO2
access to the low valent active sites. Furthermore, the protein
scaffold of the enzyme allows for a synchronized inux of
electrons and substrates enabling high enzymatic selectivity.
We and others recently showed that efficient electrocatalysts
Energy production from fossil fuels results in critical environ-
mental contaminations due to the emission of the greenhouse
gas CO2 into earth's atmosphere.1 While CO2 acts as a pollutant,
it is also a cheap and abundant C1-feedstock for carbon-based
chemicals if efficient pathways for its conversion are estab-
lished.2 Especially, the development of sustainable production
processes by means of electrocatalysis are a promising pathway
to efficiently activate CO2 and allow for the utilization of elec-
tricity from renewable sources like wind or photovoltaics.3
While electrochemical CO2 reduction was previously shown, the
hampered mass transport and low water solubility of CO2
render this process still inefficient and instead facilitate the
formation of H2.4,5 In addition, the metal–substrate binding
strength (H+ vs. CO2), and electron transfer kinetics at the
material surface, are among the key factors for the selectivity of
the electrocatalytic CO2 reduction.6,7 While low-coordinated Cu-
nanoparticle sites facilitate HER, up to a certain extent, nano-
structuring of Cu particles was shown to favor CO2 reduction
over HER.8–11 General design principles leading to more efficient
electrochemical CO2 reduction catalysts, however, are not yet
available.12 Notably, the capability of a catalyst to show either
a
¨
Inorganic Chemistry I, Ruhr-University Bochum, Universitatsstrasse 150, 44780
Bochum, Germany. E-mail: ulf.apfel@rub.de
bFraunhofer UMSICHT, Osterfelder Straße 3, 46047 Oberhausen, Germany
c
¨
Department of Physics, Ruhr-University Bochum, Universitatsstrasse 150, 44780
Bochum, Germany
dDepartment of Interface Science, Fritz-Haber Institute of the Max Planck Society,
Faradayweg 4-6, 14195 Berlin, Germany
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8sc03555e
Fig. 1 Active site in the nickel-containing CO dehydrogenase (pdb:
1su6) (a) and crystal structure of the Fe4.5Ni4.5S8 (b).
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can be designed following natural principles.24–31 In particular,
the mineral Fe4.5Ni4.5S8 (Fig. 1b) acted as a functional mimic for
[NiFe] hydrogenase and allowed for H+ reduction at over-
potentials of 190 mV at 10 mA cmꢀ2 with high stability and
without the need of any articial nano-structuring. The distinct
material composition of the electrode surface and its high
conductivity was shown to be responsible for its activity and
supported by operando phonon spectroscopy as well as scanning
electrochemical cell microscopy.32,33 Following our previous
design concepts for electrode materials and our knowledge on
the surface and bulk structural composition,26,32,33 we reasoned
that due to close structural analogy with the enzyme CO dehy-
drogenase (CODHNi), Fe4.5Ni4.5S8 should likewise act as
a potential electrocatalyst for the reduction of CO2. Here,
comparable to the CODHNi, Fe4.5Ni4.5S8 possesses distinct
[Fe,Ni] sulfur clusters and reveals Fe–Ni sites with an M–M bond
Fig. 2 (a) Linear sweep voltammograms recorded at 5 mV sꢀ1 in
saturated CO2 solutions. (b) Controlled potential coulometry (CPC)
was performed in different electrolytes (0.1 M TBAPF6) at ꢀ1.80 V vs.
NHE and (c) the composition of the CO2 reduction products analyzed
of around 2.6 A [ca. 2.8 A in CODHNi]21 that is bridged by labile
sulfur atoms. This bio-inspired design attempt is further sup-
ported by the hydrothermal hydrogenation of CO2 observed in
˚
˚
hydrothermal vents and facilitated by Fe2Ni7S8 possessing by on-line gas chromatography after 1 h. Notably, formation of soluble
a pentlandite structure.34 Considering the observed natural
performance and the close resemblance of FeNi suldes to the
reduction products was not observed in additionally performed GC
and HPLC experiments.
enzyme CO dehydrogenase, it is surprising that up to now only
little is known about the performance of such iron- and nickel-
sulde materials for CO2RR.24 Markedly, solely pyrite (FeS2)13
and greigite (Fe3S4)35–37 were previously reported to allow elec-
trochemical reduction of CO2 in water but with low overall
faradaic efficiencies (FE) for CO2 reduction (<10%) with low
stability. We herein show that Fe4.5Ni4.5S8 bulk electrodes can
efficiently facilitate CO2RR with high faradaic efficiency towards
CO formation (87 ꢁ 4%), at moderate current densities (3 mA
cmꢀ2) and potentials (ꢀ1.8 V vs. NHE). We furthermore reveal
the electrochemical behavior of common C1-reduction products
like formaldehyde, CO, or formic acid explaining the observed
product distribution.
respectively. In dimethylformamide (DMF), dimethyl sulfoxide
(DMSO) or propylene carbonate (PC) containing 0.1 M TBAPF6,
current densities above 10 mA cmꢀ2 were not observed reaching
only maximum values of ca. 7.6 mA cmꢀ2 at ꢀ2.2 V.
While certainly, the lower conductivity and with it the higher
charge transfer resistance of the organic solvents will have an
inuence on the currents, the higher proton concentration in
the more protic solvents also facilitates the H2 generation
(Fig. 2c). Notably, in aqueous KHCO3, a faradaic efficiency (FE)
for H2 formation of up to 93% was observed at ꢀ1.8 V vs. NHE
which is in line with the previously reported high activity of this
material towards HER. The contribution of the CO2RR is small
and CO, CH4 as well as C2H6 were observed with low faradaic
efficiencies of 1.0%, 0.2% and 0.1%, respectively. While at
ꢀ1.2 V vs. NHE in KHCO3 HER is still the dominating reaction,
the contribution of the CO2RR is slightly improved revealing
faradaic efficiencies of 1.2% (CO), 1.8% (CH4), 0.3% (C2H6) and
61.5% (H2). The observed performance of Fe4.5Ni4.5S8 is in good
agreement with the reports on greigite and pyrite in aqueous
conditions.13,14,35
Results and discussion
Solvent dependency of CO2RR with pentlandite
Fe4.5Ni4.5S8 was synthesized via a high-temperature route at
1100 ꢂC following literature procedures.25,38 Pellet electrodes
were prepared from the as-synthesized material, mounted into
a Teon casing.33 Their performance was subsequently tested in
different organic and aqueous electrolytes (Fig. 2a and b). While
performing linear sweep voltammetry (LSV) from 0 to ꢀ2.2 V vs.
NHE and controlled potential coulometry (CPC) at various
potentials, the electrolyte solution was continuously purged
with CO2 and the gas ow analyzed by online gas
chromatography.
Likewise, the liquid phase was analyzed via HPLC analysis to
reveal any potentially soluble reduction products. However,
neither alcohols, carboxylic acids or aldehydes were detected. In
aqueous KHCO3 (0.1 M), the bulk electrode shows a catalytic
current density of 20 mA cmꢀ2 at ꢀ1.3 V. In contrast, in
methanol, pyridine/water (0.01%) and acetonitrile (113 ppm
H2O; 0.01%), comparable catalytic currents were observed at
signicantly more negative potentials of ꢀ1.6, ꢀ1.7 and ꢀ2.1 V,
Although water is indeed the preferred electrolyte to perform
CO2RR, the obvious CO2 reduction, led us to further investigate
the role of other solvents in this process. The utilization of
organic solvents is furthermore accompanied by a rm control
of the proton concentration in the electrocatalytic double layer
leading to analogous conditions that are usually found in gas
diffusion electrode layers.39 In dry methanol, HER is still the
dominating process (FE 78%). However, the fraction of CO2RR
on the total faradaic efficiency is increased showing that the
lower acidity of methanol, and thereby the lower proton avail-
ability, facilitates CO2 reduction (Table S1†). The overall impact
of CO2RR in MeOH likewise slightly increases at more negative
potentials with total faradaic efficiencies for all detected carbon
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products of 3.2% at ꢀ1.2 V to 5.7% at ꢀ1.8 V vs. NHE. In pyri- selectivity towards CO2RR using MeCN electrolyte solutions
dine, HER is signicantly suppressed showing faradaic effi- with varying water concentrations (24–30 000 ppm) (Table S3†)
ciencies for H2 of solely 60, 30 and 8% at ꢀ1.2, ꢀ1.5 and ꢀ1.8 V. by performing LSV (Fig. 3a).
In addition, a clear increase of the FE for CH4 from solely 0.3 to
A shi of the potential necessary to achieve a catalytic
3.0% at ꢀ1.2 V was observed. In contrast, in MeCN, DMSO, DMF current of ꢀ20 mA cmꢀ2 to lower potentials (ꢀ2.1 V; 24 ppm
and PC, Fe4.5Ni4.5S8 electrodes reveal a signicantly altered H2O vs. ꢀ1.3 V; 30 000 ppm) is observed as the water concen-
behavior. In these solvents, the faradaic yields for HER are tration in the electrolyte increases. However, gas analyses aer
signicantly reduced and an increased amount of CO2RR 1 h CPC at ꢀ1.8 V clearly shows that at higher water concen-
products is observed. It is noteworthy that in MeCN and PC trations HER is facilitated and CO2RR is suppressed (Fig. 3b).
methane (FE 12.8% in MeCN, 8.3% in DMSO and 13.0% in PC) While for high water concentrations in the electrolyte, FEs of
and ethylene (FE 0.5% in MeCN, 2.2% in DMSO and 1.4% in PC) 87% (30 000 ppm) and 62% (5000 ppm) are observed for
are the major products at ꢀ1.2 V vs. NHE which corroborates hydrogen, only 11% and 19% FE, respectively, were found for
the nding of pentlandite being capable to hydrothermally CO. Contrary, in the presence of lower water concentrations,
hydrogenate CO2.34
CO2RR becomes the dominant reaction. Here, FE for CO
At very negative potentials, the CO formation capability of signicantly increases to 37% (500 ppm H2O) and 87% (24 ppm
the material is generally increased reaching FE as high as 71% H2O), respectively, concomitant with a decrease of the FE for
in MeCN, 42% in DMSO and 51% in PC at potentials of ꢀ1.8 V. HER to 14% and 3%, respectively.
Surprisingly, the product distribution in DMF is less dependent
In control experiments using Ar purged electrolytes con-
on the potential applied showing steady methane formation (FE taining 24 ppm H2O, no formation of CO2RR was observed
23%) and smaller variations in CO formation (FE 11–30%) as (Fig. S6†). In addition, to rule out a suppressed oxidation on the
compared to MeCN, DMSO and PC. While CH4 and CO are the anode side due to an imbalanced amount of charges passed
main products from CO2 reduction, it is notable that especially through the system, we added conc. sulfuric acid to the aceto-
in aprotic solvents C2H4 and C2H6 are also observed (Fig. 2c). nitrile mixture at the cathodes chamber. This allows for a fast
E.g. FE for C2H4 of 2.2% (DMSO), 0.7% (DMF) and 0.5% (MeCN) HER reaction and allows for a comparable oxidation reaction as
were observed. Likewise, C2H6 was observed in the range of 1– observed under CO2RR catalytic condition. Similarly, no
1.3%, respectively. The faradaic efficiencies below 100% increased formation of CO2 reduction products (Fig. S7†) was
observed within the non-protic solvents can most-likely be observed proving the dominant reduction of CO2 rather than
assigned to high capacitive currents and poor conductivity electrolyte decomposition. Similar results were observed when
leading to e.g. heat formation, gas leakage, as well as a potential using DMSO or DMF under otherwise identical conditions. To
partial re-oxidation of CO2 reduction products and are not conrm the origin of the various C-products observed, we per-
necessarily an effect of the electrode behavior in the different formed a mass-selective product analysis utilizing 13CO2 in
solvents.40 However, these amounts are among the highest ever acetonitrile under otherwise identical electrochemical condi-
reported for non-Cu containing materials (Table S4†).41 We tions. While the observed mass peak of 29 is indicative for the
conclude that beside the proton availability and the solvent formation of 13CO from 13CO2 (Fig. S9†), the unaltered mass
chosen, the CO2 solubility in the various solvents has a major
inuence on the CO2RR by enhancing the effective concentra-
tion of CO2 at the electrode surface.
The solubility of CO2 in acetonitrile (0.27 M) was reported to
be signicantly higher compared to water (0.04 M) and meth-
anol (0.16 M).42 Likewise, an increased CO2 solubility is
observed for DMF (0.18 M)43 and PC (6.97 M)43 which encour-
ages the CO2 reduction in these solvents (Table S5†). However,
the solubility of CO2 in DMSO is just 0.02 M and contradicts this
hypothesis. In this regard, the proton availability which is
commonly displayed by the acidity of water in a particular
electrolyte or the solvent itself will inuence CO2RR and is
highlighted by increased HER activity in aqueous KHCO3 (pKa
10.3) compared to MeOH (pKa 16).
Dependency on water/proton concentration
The efficiency of CO2RR seems to be a compromise between
altered proton concentration at the electrode surface (generally
caused by the water content in a given solvent) and the capa-
Fig. 3 Linear sweep voltammograms recorded at 5 mV sꢀ1 in MeCN
with different water content and CO2 purge (a), faradaic efficiencies of
CO2 reduction products in the gas phase at a constant potential of
bility of a certain electrolyte to efficiently absorb CO2. We thus
ꢀ1.8 V vs. NHE from MeCN with various amounts of water (b). Long
opted to determine the inuence of the water amount on the
term electrolysis for 15 h at ꢀ1.8 V vs. NHE with 24 ppm water
activity of Fe4.5Ni4.5S8 electrodes. Hence, we examined the including the observed faradaic efficiencies (c).
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pattern of ethane suggests a different origin (Fig. S10†). We thus state of the pentlandite catalysts as-prepared and aer CO2RR.
repeated the CO2 reduction experiment in the presence of Both show a signicant content of oxygen as it can be seen from
CD3CN. The resulting mass pattern and the observed mass peak the survey scans (Fig. S12†). Moreover, the sample aer reaction
at 33 clearly indicate the formation of C2D3H3 and suggest that bears considerably high amounts of the electrolyte residuals as
ethane originates from the hydrogenation of acetonitrile rather indicated by intense F 1s, N 1s, and C 1s peaks. A closer
than reduction of CO2 (Fig. S10†). Likewise, we analyzed the investigation of the high-resolution regions of the metallic
origin of CH4 that was observed within our experiments via the components (Fig. 4a and b) and sulfur (Fig. 5) conrms severe
same technique. While the MS pattern of CH4 overlaps with oxidation of the catalyst surface before and aer the reaction
those of the reaction matrix under the applied instrumental and subsequent exposure to air. A spectral deconvolution of the
conditions, increase of the peak intensity at m/z ¼ 17 and the Fe 2p3/2 photoemission line using an Fe(III) multiplet structure45
missing higher mass peaks for CD3H and derivatives upon and a single FeSx peak (Fig. 4c) indicates only 18 at% of the
performing the experiments in CD3CN indicate that likewise latter.
CH4 is formed from CO2 (Fig. S11†).
Similarly, the Ni 2p3/2 region is dominated by the Ni(OH)2
Notably, ne-tuning of the water concentration in the elec- structure46 represented by a photoemission peak at 855.9 eV and
trolysis cell and the applied potential allows for the tailored a characteristic shake-up satellite at 861.4 eV (Fig. 4d). The peak
electrochemical production of syngas (Table S4†). Markedly, for at 853.0 eV represents a sulde species and corresponds to
CH4 and C2H6 that were likewise found as reduction products, ꢃ23% of the total Ni content. XPS analysis of the sample aer
FE is independent of the water concentration (Fig. S1†) sug- reaction does not display a signicant change in the chemical
gesting a different catalytic mechanism. The results clearly state of Fe and Ni, although one needs to consider that both
show that HER and CO2RR are both catalyzed by Fe4.5Ni4.5S8 samples were measured ex situ and that at least partial catalyst
electrodes and the outcome of the reduction can be tuned by the reduction is expected under reaction conditions that can be
process conditions rather than by further catalyst modication. further re-oxidized upon ex situ sample transfer in air. More-
over, calculation of the Fe : Ni ratio for the sulde species,
shows enrichment with nickel before and aer reaction. The S
Durability and potential intermediates
2p region of both samples shows a shape typical for FexNi9ꢀxS8
To further investigate the Fe4.5Ni4.5S8 bulk electrodes for its materials25 with a structure between 160–165 eV, characteristic
capability to act as a potent electrocatalyst in CO2RR, we per- of oxysulde lms,47 and
a mildly oxidized iron pyrite
formed stability tests in acetonitrile (24 ppm water H2O) at an surface.48,49 The structure at higher binding energy is deconvo-
applied potential of ꢀ1.8 V over 15 h (Fig. 3c). During the luted in two doublets, at 168.4–169.7 and 166.1–167.3 eV, rep-
electrolysis, the system operated at a constant current density of resenting sulfate and sulte species correspondingly.50 The
around 3 mA cmꢀ2. While in the beginning of the controlled structure at higher binding energy is deconvoluted in two
potential electrolysis, high faradaic efficiencies of up to 87% CO doublets, at 168.4–169.7 and 166.1–167.3 eV, representing
are observed, it decreases slowly with electrolysis time to 75%. sulfate and sulte species, correspondingly. Remarkably, the
In equal measure, the FE for H2 increases from 8 to 21%. This amount of oxidic sulfur species increases from 17 to 50 at%
change in the CO:H2 selectivity can be caused by either an aer the reaction. Despite the high amount of oxidized species
incremental change of the catalyst surface and hence its reac- observed by XPS, we do not indent to identify those as active in
tivity or an increase in water concentration over the course of
the electrolysis. Notably, SEM-EDX showed only carbon depo-
sition. The Fe:Ni:S content, however, is not affected, indicating
high stability of the material. We subsequently quantied the
water content aer 15 h electrolysis and observed an elevated
water concentration, responsible for the hampered catalyst
performance. This increase most likely originates from traces of
water present in the CO2 stream causing accumulation of water
in the electrolyte over time. To further evaluate the stability of
Fe4.5Ni4.5S8, we performed long-time stability experiments in
the presence of various amounts of water. Noteworthy, aer
conducting electrolysis at ꢀ1.8 V in a 5000 ppm water acetoni-
trile solution, the electrode is covered with a black lm and
correlates with the observed decrease of CO2RR activity. Energy
dispersive X-ray spectroscopy (EDX) revealed a high carbon
content of up to 80% at the electrode surface (Fig. S2†). While
similar results were previously observed for Cu electrodes,44 the
exact reason of this electrode passivation, however, is not well
understood and remains subject of future investigations.
Fig. 4 (a) Fe 2p and (b) Ni 2p XPS spectra of pentlandite before and
after CO2RR. Examples of the spectral deconvolution fit for the as-
prepared sample are shown in (c) for the Fe 2p3/2 core level and (d) for
Ex situ X-ray photoelectron spectroscopy (XPS) measure-
ments were carried out to reveal the composition and chemical Ni 2p3/2
.
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ꢀ0.10 and ꢀ0.28 mA cmꢀ2 under otherwise identical conditions
(Fig. S3†). Analysis of the gaseous products aer 1 hour at
ꢀ1.2 V and ꢀ1.5 V, revealed signicant higher HER activity of
Fe4.5Ni4.5S8 with FEs of 34 and 51%, respectively, for H2 in
comparison to those observed for Cu bulk electrodes (0 and
6%). In lieu, Cu revealed higher efficiency towards methane
formation with FE of 20 and 9% at ꢀ1.2 V and ꢀ1.5 V respec-
tively versus the observed FE of 13 and 4% for Fe4.5Ni4.5S8
electrodes under identical conditions. Likewise, the efficiency
for CO at Cu electrodes (FE 39%, ꢀ1.2 V; FE 9%, ꢀ1.5 V) is
higher than observed for Fe4.5Ni4.5S8 (FE 11.4%, ꢀ1.2 V; FE 4%,
ꢀ1.5 V). Notably, these differences are only observed at low
current densities/low potentials. At ꢀ1.8 V, there is only a small
difference in the product distribution. Both, Fe4.5Ni4.5S8 and Cu
electrodes reveal suppressed H2 formation and facilitate the
CO2 reduction to mainly CO (72% FE for both electrodes) and
traces of methane (1% FE for Fe4.5Ni4.5S8 and 6% FE for Cu).
Notably, at such high potential conditions, Fe4.5Ni4.5S8 gener-
ally reveals higher current densities suggesting faster conver-
sion kinetics and revealing its potential (Fig. S3†).
Fig. 5 S 2p XPS spectra of the pentlandite before and after CO2RR (a)
and spectral deconvolution exemplified for the as-prepared sample
(b). The decrease in the intensity of the S signal observed after reaction
is at least partially related to the overlayer resulting from electrolyte
residues.
The overall narrow product distribution of Fe4.5Ni4.5S8
pointed our attention towards potential intermediates of the
CO2RR and their subsequent conversion on Fe4.5Ni4.5S8 bulk
electrodes. We thus tested the reactivity of various potential CO2
reaction products, such as carbonate, CO, formaldehyde, formic
acid and oxalic acid under electrocatalytic conditions (at ꢀ1.8 V
vs. NHE) in acetonitrile containing 24 ppm water.
CPC in saturated Cs2CO3 solutions (3.31 mM) in MeCN
yielded exclusively CO as the major product (42% FE, Fig. 6a)
and suggests that likewise, carbonates can serve as a CO2
source. In the presence of oxalic acid, the major reduction
product is CO (FE 3.9%) and only trace amounts of hydroge-
nation to afford C2H4 (FE 0.05%), C2H6 (FE 1.60%) is also
detected. Additionally, CO poisoning experiments were per-
formed since Fe and Ni strongly adsorb CO and usually undergo
surface deactivation. CPC experiments at ꢀ1.8 V vs. NHE in
a solution of MeCN (24 ppm H2O, Fig. S4 and S5†) applying
a continuously applied CO stream showed no obvious inuence
on the catalytic current and suggests no CO poisoning.
Furthermore, neither gaseous nor liquid CO conversion prod-
ucts were detected, underlining the inertness of the Fe4.5Ni4.5S8
electrodes towards further CO conversion. Under same condi-
tions, a 37% formaldehyde solution afforded high amounts of
CH4 (FE 4.2%) and traces of C2H4 (FE 0.13%) and C2H6 (FE
0.10%) (Fig. 6b). Notably, only negligible amounts of CO were
observed herein. It is worth to highlight that C2-reduction
products are observed in the presence of oxalic or formic acid
(Fig. 6c and d) although with low efficiency (FE < 1%). Our
results clearly show that although the formation of alternative
reduction products seems possible, they will be converted to
mainly CO and methane with a minor amount of C–C coupling
products.
Fig.
6 GC traces and recorded current densities starting from
different carbon sources like Cs2CO3 (a), formaldehyde (b), oxalic acid
(c) and formic acid (d).
the CO2RR. Both Fe and Ni are well known to form native oxide
overlayers upon exposure to air. However, the increased amount
of oxidic sulfur species is striking. We attribute it to the
formation of S defect sites under reaction conditions, which are
then more prone to oxidation when exposed to air. Notably,
such [NiFe]-oxido-species are also found in the natural arche-
type Ni-dehydrogenases and are discussed as key intermediates
in the enzymatic CO2 reduction.51,52
As copper is one of the benchmark materials in CO2RR, we
decided to compare Fe4.5Ni4.5S8 and a copper disc electrode
under the same experimental conditions. CPC experiments at
ꢀ1.2, ꢀ1.5 and ꢀ1.8 V vs. NHE were subsequently performed in
acetonitrile (113 ppm H2O) (Fig. S3†). While we observed
current densities of ꢀ0.14, ꢀ0.73 and ꢀ3.2 mA cmꢀ2 at ꢀ1.2,
ꢀ1.5, and ꢀ1.8 V, respectively, for Fe4.5Ni4.5S8, Cu bulk elec-
trodes showed signicantly smaller current densities of ꢀ0.01,
Conclusion
In conclusion we have shown, that Fe4.5Ni4.5S8, much like its
natural congeners, [FeNi]-hydrogenase and CODHNi, can
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catalyze HER as well as CO2RR depending on the chemical
environment and with seemingly comparable catalytic inter-
mediates. In this regard, protic electrolytes such as aqueous
9 L. Dai, Q. Qin, P. Wang, X. Zhao, C. Hu, P. Liu, R. Qin,
M. Chen, D. Ou, C. Xu, S. Mo, B. Wu, G. Fu, P. Zhang and
N. Zheng, Sci. Adv., 2017, 3, e1701069.
KHCO3, methanol as well as aprotic pyridine promote HER with 10 H. S. Jeon, S. Kunze, F. Scholten and B. Roldan Cuenya, ACS
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I. Sinev, Y.-W. Choi, K. Kisslinger, E. A. Stach, J. C. Yang,
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simple iron and nickel-based electrocatalysts.
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Conflicts of interest
There are no conicts to declare.
19 R. Francke, B. Schille and M. Roemelt, Chem. Rev., 2018, 118,
4631–4701.
Acknowledgements
20 G. Zhao, X. Huang, X. Wang and X. Wang, J. Mater. Chem. A,
2017, 5, 21625–21649.
We thank Dr M. Reback for valuable hints during manuscript
preparation. The authors thank for the nancial support of the
Fonds of the Chemical Industry (Liebig grant to U.-P. A.), the
Deutsche Forschungsgemeinscha (Emmy Noether grant to U.-
P. A., AP242/2-1 and AP242/6-1) as well as the Fraunhofer
Internal Programs under Grant No. Attract 097-602175. IS and
BRC also acknowledge the nancial support of the German
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