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Cite This: J. Phys. Chem. Lett. 2019, 10, 513−517

Selective Electrochemical Nitrogen Reduction Driven by Hydrogen
Bond Interactions at Metal−Ionic Liquid Interfaces
Manuel A. Ortuño,*,† Oldamur Hollóczki,‡ Barbara Kirchner,‡ and Núria López*,†
†

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Institute of Chemical Research of Catalonia (ICIQ) and Barcelona Institute of Science and Technology (BIST), Av. Països
Catalans 16, 43007 Tarragona, Spain
‡
Institut für Physikalische und Theoretische, Universität Bonn, Mulliken Center for Theoretical Chemistry, Beringstrasse 4−6,
53115 Bonn, Germany
S
* Supporting Information

ABSTRACT: Increasing the activity of the nitrogen reduction reaction while slowing the
detrimental hydrogen evolution reaction is a key challenge in current electrocatalysis to provide
a sustainable route to ammonia. Recently, nanoparticles in ionic liquid (IL) environments have
been found to boost the selectivity of electrochemical synthesis of ammonia from dinitrogen at
room temperature. Here, we use for the ﬁrst time a fully atomistic representation of metal−IL
interfaces at the density functional theory level to understand experimental evidence, with
particular focus on the rate and selectivity determining formation of N2H intermediates
compared to hydrogen evolution. We ﬁnd that decorating the metal surface with ﬂuorinated ILs
creates speciﬁc H-bond interactions between Ru−N2H and IL anions, stabilizing this
intermediate and thus driving the selectivity of the electrochemical process.

A

mmonia synthesis, one of the most important processes in
the chemical industry, keeps the supply of fertilizers and
related N-containing chemicals worldwide. The Haber−Bosch
process, which involves the thermally driven hydrogenation of
N2 under harsh pressure and temperature conditions, produces
500 million tons of NH3 per year and consumes ca. 1% of the
global energy.1 Transition from Haber−Bosch to less energydemanding processes is a major goal in chemistry.2,3
Mimicking the gentle reaction conditions found in enzymes,
electrocatalysis is a promising alternative to achieve the
nitrogen reduction reaction (NRR) at low temperature.4−6
The challenge is promoting the NRR while shutting down the
competing hydrogen evolution reaction (HER) that reduces
the eﬃciency of the process.7 However, the rational design of
new only metal-based catalysts with enhanced selectivity is
limited by how the stabilities of certain intermediates are
dependent on each other, in other words, by linear scaling
relationships.8,9
Experimental eﬀorts have boosted Faradaic eﬃciencies by
working in a molten hydroxide suspension at high temperature10 and supporting Au nanoparticles on CeOx−reduced
graphite oxide.11 Quite recently, MacFarlane and coworkers
reported Fe-based catalysts with a Faradaic eﬃciency of 32%12
and 60%13 when using ﬂuorinated ionic liquids (ILs). Besides
the relatively higher solubility of N2 in ILs,14 the speciﬁc role of
ILs on the selective NRR is not fully understood. To shed light
into these features, we turn to theoretical methods.15
Herein, we employed density functional theory (DFT) to
gain mechanistic insight on IL-decorated metal surfaces16 for
the electrochemical reduction of N2 to NH3 (Figure 1a). Such
metal−IL interfaces have the potential to circumvent linear
scaling relationships, which opens the path to enhanced NRR
© 2019 American Chemical Society

Figure 1. (a) Ru-catalyzed electrochemical nitrogen reduction
reaction on clean (dotted square) and IL-decorated (striped square)
surfaces. (b) N-Butyl-N-methylpyrrolidinium [C4C1pyrr]+ and tris(perﬂuoroethyl)-triﬂuorophosphate [FAP]− under study.

selectivity. We modeled a Ru-based catalyst, as studied in
literature,17 and the IL [C4C1pyrr][FAP] (Figure 1b), as
reported experimentally.13 Computational studies on bulk ILs
have been extensively reported,18 but due to its complexity,
less attention has been paid to their interface with solids.19,20
Pádua and coworkers21 parametrized metal−IL interactions to
study the solvation of Ru nanoparticles, and Heinz et al.22
performed classical molecular dynamics (MD) simulations on
Au−IL structures. To the best of our knowledge, this is the ﬁrst
fully atomistic study of a bulk IL on a transition metal surface
at DFT level that directly addresses reactivity.
Received: November 8, 2018
Accepted: January 15, 2019
Published: January 15, 2019
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Letter

between the N atom and the inner F−CF atoms are less
marked than those involving the more accessible F−CF2 and
F−P atoms (Figure S4). Additional RDF data from classical
and ﬁrst-principles MD simulations with 24 IL pairs can be
consulted in Figures S5−6.
Next, we explored the interface between the Ru catalyst and
the IL. For that purpose, we designed a 3-layer p(6 × 6)
Ru(0001) slab with 8 IL pairs equilibrated through classical
and ﬁrst-principles MD simulations. The use of isolated IL
pairs would not capture the bulk eﬀect of the liquid; likewise,
continuum methods would not describe explicit interactions
between solute and solvent. The aim here is to describe the IL
as a bulk liquid rather than as isolated pair clusters. The ﬁnal
structure after equilibration is shown in Figure 3a. Close

First, to evaluate the dynamic behavior of the ﬂuorinated IL
at hand, we described the liquid phase of the pure IL,23 N2@
IL, and NH3@IL by means of classical MD simulations. We
used a cubic box with 240 IL pairs to model the bulk phase
(Figure 2a). For N2@IL and NH3@IL, we included one N2 or

Figure 3. First-principles MD simulations. (a) Repeating unit cell of
the interface formed by a 3-layer p(6 × 6) Ru(0001) slab with 8 IL
pairs (548 atoms) and (b) zoom-in insets highlighting Ru···F (top)
and Ru···H (bottom) close contacts. Legend: Ru (green), P (orange),
F (pink), N (blue), C (gray), H (white).

Figure 2. Classical MD simulations. (a) Cubic box containing 240 IL
pairs (cations in gray, anions in purple) and (b) corresponding RDFs
for the NH3@IL simulation, highlighting contacts between the H
atoms of NH3 and the F atoms of [FAP]−.

contacts between the Ru surface and the F−CF2 group in
[FAP]− (Ru···F distance of 3.026 Å) and the H−CH2 group of
the butyl chain in [C4C1pyrr]+ (Ru···H distance of 2.363 Å)
are depicted in Figure 3b.
With an atomistic representation of the clean metal and the
metal−IL interface at hand, we next evaluated the potential
energy surface of the nitrogen reduction and hydrogen
evolution reactions at DFT level. Species involving the clean
surface are denoted with A−E. The corresponding ILdecorated counterparts follow the same nomenclature and
add the preﬁx IL. Due to the rich chemical composition of the
interface (Figure 3b), we found several conﬁgurations for each
intermediate, but only the most stable ones are reported here.
The explicit representation of the solvent on clean metal
surfaces24 is out of the scope of the present work, which rather
focuses on the selectivity induced by ILs.
Guided by previous computational studies on Ru
catalysts,8,25−27 we focused on the adsorption of N2 and
formation of the N2H intermediate, a key rate- and selectivitydetermining step (additional intermediates can be consulted in
Table S1) and also computed the competitive hydrogen

one NH3 molecule per simulation box, respectively. Radial
distribution function (RDF) data, which show probable atom−
atom distances, can be consulted in Figures S2−4 for all three
simulations. RDF data for the N2@IL simulation (Figure S3)
show signiﬁcant contact between N2 and the methyl C atom of
the butyl chain of [C4C1pyrr]+, suggesting that N2 might be
mainly in the nonpolar microphase. The most relevant RDF
data for the NH3@IL simulation are summarized in Figure 2b.
At ∼3 Å, one can observe interplays between the N atom of
NH3 and (i) the F atoms of the CF3 groups (F−CF2) in
[FAP]− anions (solid red line) and (ii) the F atoms directly
attached to P (F−P) in [FAP]− anions (solid blue line).
Concurrently, some interactions can be detected starting at ∼2
Å between the H atoms of NH3 and F−CF2 and F−P atoms
(dashed red and blue lines, respectively). The occasional close
approach of the H and F atoms results in a shoulder on the
RDFs and points to the possibility of a weak H-bond
interaction between NH3 and [FAP]−. The interactions
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increases both the activity (by stabilizing IL-C compared to C)
and selectivity (by stabilizing IL-C compared to IL-E). Such
eﬀect is better displayed in Figure 4c, which represents how
the selectivity to HER for the non-IL system (left) switches to
NRR in the presence of IL (right). The eﬀect of the IL on
selectivity remains the same when considering zero point
vibrational energy corrections and Gibbs energies (Table S3).
The above-mentioned results can be explained by looking at
the atomic structure of the N2H intermediate in Figure 5a.

evolution to address selectivity (Figure 4a). Figure 4b displays
the relative electronic energies of key intermediates during

Figure 5. DFT-optimized structures of (a) the N2H intermediate, ILC, and (b) the H2 intermediate, IL-E, both including an IL
conﬁguration coming from the metal−IL MD simulations. Selected
ILs are omitted for clarity. The yellow dashed lines highlight H···F
interactions. Legend: Ru (green), P (orange), F (pink), N (blue), C
(gray), H (white).
Figure 4. (a) Rate and selectivity determining step comparison
studied herein and (b) corresponding reaction energy proﬁles of such
intermediates during NRR (right) and HER (left) for clean (dashed
green line) and IL-decorated (solid purple line) Ru surfaces. (c)
Relative energy diﬀerences for NRR (circles) and HER (triangles)
according to the absence (green) and presence (purple) of IL. The
change of selectivity from HER to NRR is indicated with a yellow
circle. Lines (solid for NRR, dashed for HER) serve as a guide to the
eye.

Compared to the Ru-only surface C, IL-C exhibits a clear
interaction between the N2H group and the F−CF2 atom in
[FAP]−. The H···F and N···F distances of 1.865 and 2.871 Å,
respectively, evoke the patterns found in the liquid NH3@IL
simulation (Figure 2b). Likewise, similar contacts might also
be at work on N2Hx species28 or dissociative mechanisms
involving NHx intermediates.25−27 Binding energies of ca. 0.1
eV have been estimated for related contacts involving CH3OH
and CH3CF3.29 Our relative ΔE of 0.34 eV between B and C
(0.64 eV) and IL-B and IL-C (0.30 eV) suggests that this
interaction alone might not be responsible for the stabilization
and that the inherently charged bulk IL environment has an
additional electrostatic eﬀect30−32 and should not be ignored
in the modeling. As for the competing HER, the H2
intermediate IL-E is shown in Figure 5b. The interaction
between the nonpolar H2 and F atoms is weaker (H···F
distance of 2.471 Å), and thus, the stabilization is less marked
compared to N2H species. Overall, H···F interactions in ILcontaining NRR intermediates boost the selectivity of the
process, in line with recent experiments.13
With respect to the role of reactant water, it has been shown
that (i) the OH groups of hexaﬂuoroisopropanol cluster
together, forming polar domains, and are separated from the
ﬂuorine-based groups,33 and (ii) NH-based species with a free
lone pair interact with water as H-bond acceptors rather than
H-bond donors. This suggests that N2H···F interactions, in
which N2H acts as H-bond donor (Figure 5a), will be probably
not disturbed. In agreement with this discussion, experiments
show that the concentration of water (from 20 to 250 ppm)
did not strongly impact the selectivity of the reaction.13

NRR (right side) and HER (left side). On the one hand, the
dashed green line shows the energy values for the clean Ru
surfaces. The end-on adsorption of N2 on A giving rise to B is
exothermic by 0.87 eV (the side-on adsorption is only 0.23 eV
downhill). Subsequent formation of the N2H intermediate C
demands 0.64 eV above B. Regarding HER, the reaction from
D, adsorbed H, to E, physisorbed H2, requires 0.52 eV. The
lower relative energy of HER (0.52 eV) compared to that of
NRR (0.64 eV) explains the low selectivity toward ammonia
observed for pure metal catalysts. On the other hand, the solid
purple line shows the energy values for the IL-decorated Ru
surfaces. The end-on adsorption of N2 on IL-A to form IL-B is
also exothermic by 0.83 eV (the side-on adsorption is only 0.20
eV downhill), suggesting that the IL surroundings play a small
role at this stage. However, the formation of the N2H
intermediate IL-C is signiﬁcantly favored, requiring only 0.30
eV comapred to 0.64 eV for the clean surface. As for HER, the
H2 formation from IL-D to IL-E demands 0.38 eV. This value
is lower than that for HER with the clean surface, 0.52 eV, but
slightly higher than that for NRR with the IL-decorated
surface, 0.30 eV. These results show that the IL environment
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In conclusion, we followed an elaborated computational
protocol to model metal−ionic liquid interfaces at the DFT
level. Particularly, we designed a Ru(0001) surface in contact
with 8 pairs of [C4C1pyrr][FAP]. We used the resulting model
to study the key rate and selectivity determining N2H
intermediate in the NRR compared to the HER involved in
the electrochemical synthesis of ammonia at room temperature. Our results indicate that interactions between Ru−N2H
species and F atoms of the [FAP]− anion increase activity and
boost selectivity toward ammonia. Such fully atomistic
computational approach holds promise to unravel new
mechanistic features and guide the rational design of novel
electrocatalytic interfaces for ammonia synthesis.

■

ASSOCIATED CONTENT

S
* Supporting Information

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpclett.8b03409.
Computational details, radial distribution functions,
intermediates, energies (PDF)
Cartesian coordinates (XYZ)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: mortuno@iciq.es.
*E-mail: nlopez@iciq.es.
ORCID

Manuel A. Ortuño: 0000-0002-6175-3941
Oldamur Hollóczki: 0000-0003-3105-7176
Barbara Kirchner: 0000-0001-8843-7132
Núria López: 0000-0001-9150-5941
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
M.A.O. and N.L. acknowledge MINECO (CTQ2015-68770)
and “Juan de la Cierva−Incorporación” postdoctoral program
(IJCI-2016-29762) for ﬁnancial support. The work has been
peformed under the Project HPC-EUROPA3 (INFRAIA2016-1-730897), with the support of the EC Research
Innovation Action under the H2020 Programme. M.A.O.
gratefully acknowledges the computer resources (Cray XC40
Hazel Hen) and technical support provided by HLRSStuttgart. M.A.O. also acknowledges the computer resources at
Caesaraugusta and technical support provided by BIFIUniversity of Zaragoza (RES-QCM-2018-3-0005).

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