Enhanced Nitrate-to-Ammonia Activity on Copper-Nickel Alloys via Tuning of Intermediate Adsorption

Article abstract of DOI:10.1021/jacs.9b13347

Electrochemical conversion of nitrate (NO₃^-) into ammonia (NH₃) recycles nitrogen and offers a route to the production of NH₃, which is more valuable than dinitrogen gas. However, today's development of NO₃^- electroreduction remains hindered by the lack of a mechanistic picture of how catalyst structure may be tuned to enhance catalytic activity. Here we demonstrate enhanced NO₃^- reduction reaction (NO₃^-RR) performance on Cu₅₀Ni₅₀ alloy catalysts, including a 0.12 V upshift in the half-wave potential and a 6-fold increase in activity compared to those obtained with pure Cu at 0 V vs reversible hydrogen electrode (RHE). Ni alloying enables tuning of the Cu d-band center and modulates the adsorption energies of intermediates such as *NO₃^-, *NO₂, and *NH₂. Using density functional theory calculations, we identify a NO₃^-RR-to-NH₃ pathway and offer an adsorption energy-activity relationship for the CuNi alloy system. This correlation between catalyst electronic structure and NO₃^-RR activity offers a design platform for further development of NO₃^-RR catalysts.

Full text of DOI:10.1021/jacs.9b13347

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Article
Enhanced nitrate-to-ammonia activity on copper-
nickel alloys via tuning of intermediate adsorption
Yuhang Wang, Aoni Xu, Ziyun Wang, Linsong Huang, Jun Li, Fengwang Li, Joshua
Wicks, Mingchuan Luo, Dae-Hyun Nam, Chih-Shan Tan, Yu Ding, Jiawen Wu, Yanwei
Lum, Cao-Thang Dinh, David Sinton, Gengfeng Zheng, and Edward H. Sargent
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b13347 • Publication Date (Web): 02 Mar 2020
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Enhanced nitrate-to-ammonia activity on copper-nickel alloys
via tuning of intermediate adsorption
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Yuhang Wang†,1, Aoni Xu†,1, Ziyun Wang†,1, Linsong Huang2, Jun Li1,3, Fengwang Li1, Joshua
Wicks1, Mingchuan Luo1, Dae-Hyun Nam1, Chih-Shan Tan1, Yu Ding2, Jiawen Wu2, Yanwei
Lum1, Cao-Thang Dinh1, David Sinton3, Gengfeng Zheng2, Edward H. Sargent*,1
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1Department of Electrical and Computer Engineering, University of Toronto, 10 King’s
College Road, Toronto, ON, M5S 3G4, Canada.
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2Laboratory of Advanced Materials, Department of Chemistry, Fudan University, Shanghai
200438, China.
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3Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s
College Road, Toronto, ON, M5S 3G8, Canada.
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† These authors contributed equally to this work.
(*)
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Correspondence and requests for materials should be addressed to Edward H. Sargent
(ted.sargent@utoronto.ca) (E.H.S.)
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Abstract
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Electrochemical conversion of NO3 into ammonia (NH3) recycles nitrogen and offers
a route to NH3 production that is more valuable than dinitrogen gas. However, today’s
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development of NO3 electroreduction remains hindered by the lack of a mechanistic
picture of how catalyst structure may be tuned to enhance catalytic activity. Here we
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demonstrate enhanced nitrate reduction reaction (NO3 RR) performance on Cu50Ni50
alloy catalysts, including a 0.12 V upshift in the half-wave potential and a 6-fold increase
in activity compared to pure Cu at 0 V vs. reversible hydrogen electrode (RHE). Ni
alloying enables tuning of the Cu d-band center and modulates the adsorption energies of
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intermediates such as *NO3 , *NO2, and *NH2. Using density functional theory (DFT)
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calculations, we identify a NO3 RR-to-NH3 pathway and offer an adsorption energy-
activity relationship for the CuNi alloy system. This correlation between catalyst
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electronic structure and NO3 RR activity offers a design platform for further
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development of NO3 RR catalysts.
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Introduction
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Human activities to an anthropogenically-induced increase over time in the concentration
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of environmental nitrate (NO3 ):1, 2 the combustion of fossil fuels emits nitrous oxides (NOx);
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fertilizer-intensive agriculture releases NO3 into soil and groundwater; and NO3 -containing
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waste is discharged from industrial sources. The accumulation of NO3 induces acid rain and
photochemical smog3, 4 and the uptake of NO3 in mammals results in its in vivo conversion to
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nitrite (NO2 ),5, 6 a cause of methemoglobinemia and a known carcinogen.
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Closing the nitrate-nitrogen cycle1, 2, 7, 8 is therefore of interest: it is desirable to transform
nitrates to harmless9-11 or – better yet – to value-added products.12, 13 The electrochemical
reduction of nitrate provides a route to ammonia production,9, 13 with widespread use as a
fertilizer precursor, chemical feedstock and fuel:
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NO3 + 6H2O + 8e- → NH3 + 9OH-, E0 = 0.69 V vs. RHE (pH = 14)14 (1)
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Achieving higher-performance NO3 RR electrocatalysts remains challenging, in
significant part because the relationship between catalyst structure and activity is poorly
understood.
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To date, Faradaic efficiencies (FE) greater than 90% for NO3 reduction to NH3 have been
achieved on Cu-based catalysts, but typically these require potentials more negative than -0.27
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V vs. RHE in NO3 -containing 1 M KOH electrolytes, corresponding to an overpotential
exceeding 0.96 V.15-19 Previous studies found that alloying Cu with Ni results in a positive shift
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in the NO3 RR half-wave potential (E1/2), the potential at which the current is equal to one half
of the mass transfer limiting current, by ~0.1 V,18, 19 a finding that corresponds to enhanced
catalytic activity at a given potential. A mechanism wherein Cu performs the adsorption of
NO3* and Ni is the binding site of H* was proposed by Simpson and Johnson;20 however, this
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mechanism does not explain enhanced NO3 RR activity on CuNi alloys: Ni converts NO3 to
NH3 inefficiently,19 so that replacing surface Cu atoms with Ni would be expected to reduce
the density of active sites for NH3 production.
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We noted that upshifts in the half-wave potential (E1/2) in reduction reactions typically
suggest an increase in electrocatalytic activity. This, we posited, could arise due to a modulated
intermediate adsorption energy in CuNi alloys compared to pure Cu. Drawing parallels with
oxygen reduction reaction (ORR) literature, we noted that platinum-nickel (PtNi) alloy
catalysts typically exhibit an upshifted half-wave potential of ~0.1 V compared to pure Pt
catalysts.21, 22 Decreasing adsorption energies for oxygenated species on PtNi alloys leads to
increased activity, an instance of scaling relations for ORR catalysts.23 This has been associated
with the shifted d-band center position and surface atomic arrangement reported by Marković
and co-workers.22
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In sum, the introduction of heteroatoms modulates the electronic structure of catalysts,
enabling the enhancement of electrocatalytic activity.22-25. We explore herein how such a
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strategy can be employed to design catalysts exhibiting enhanced NO3 RR activity and
selectivity.
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We began by preparing a series of CuNi alloys with various Cu:Ni compositions, and
observed a 0.12 V upshift in E1/2, and a 0.2 V lower overpotential required for peak NH3 FE.
This occurred at a composition - Cu50Ni50 alloy catalysts - that simultaneously produced a 6-
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fold increase in NO3 RR activity compared to the case of pure Cu at 0 V vs. RHE (pH = 14).
We utilized X-ray photoelectron spectroscopy (XPS), operando X-ray adsorption spectroscopy
(XAS) and ultra-violet photoelectron spectroscopy (UPS) to investigate the electronic structure
of the catalysts and found that the Cu d-band center upshifted toward the Fermi level in CuNi
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alloys. In addition, we investigate the reaction pathway with DFT calculations, and build an
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intermediate adsorption energy-NO3 RR performance relationship for the CuNi alloy system.
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Results and discussion
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Catalyst synthesis and characterization. We began by electrodepositing catalysts on
both rotating disk electrodes (RDEs) and polytetrafluoroethylene (PTFE) membranes covered
with 300 nm thick Cu seed layers (300 nm Cu/PTFE). We used electron microscopy to
investigate the morphological and crystalline structure of the catalysts. The CuNi catalysts with
Cu-to-Ni ratios of 80:20, 50:50, and 30:70 in the deposition solutions, labeled Cu80Ni20,
Cu50Ni50, and Cu30Ni70, exhibited dendritic morphologies (Fig. 1a, Fig. S1, and S2) with
dendrite diameters in the range of 200-400 nm. Using high resolution transmission electron
microscopy (HRTEM) we observed lattice spacings of 0.208 and 0.179 nm for the Cu(111)
and Cu(200) facets (Fig. 1b) of the Cu50Ni50 catalysts due to the formation of the CuNi alloy
phase; compared to pure Cu dendrites exhibiting lattice spacings of 0.210 and 0.181 nm for
Cu(111) and Cu(200) facets that agree with cubic Cu (Fig. 1d).26 The Cu-to-Ni ratio in the
Cu50Ni50 catalyst, quantified by electron energy loss spectroscopy (EELS, Fig. 1e-h), was
~52:48. We observed similar ratios at other randomly-selected positions (Fig. S3 and Table S1),
arguing against a main role for catalyst heterogeneity in catalytic performance. XRD reveals a
decrease in Cu lattice spacings when Ni is incorporated (Fig. 1i and S4). The Cu(111) and
Cu(100) d-spacings of the Cu50Ni50 catalyst revealed by XRD agrees with the results observed
with HRTEM.
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We also looked for evidence of changes in the electronic properties of the Cu and Cu:Ni
catalysts. XPS indicated a notable decrease in the Cu2p binding energy and an increase in the
metallic Ni2p binding energy for the alloyed catalysts (Fig. 1j and S5). Among the three alloyed
catalysts, we found the largest Cu2p binding energy shift of ~0.35 eV in Cu50Ni50. This can be
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explained through electron redistribution27 which leads to an opposite shift of the Cu3d band
toward the Fermi level,28 tuning the adsorption energy of both H* and NO3*.
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In the case of the pure Ni catalysts, only the XRD peaks of the Cu/PTFE support were
observed (Fig. 1i). XPS measurements exhibited a very similar Cu2p binding energy compared
to pristine Cu/PTFE (Fig. S6). SEM, elemental mapping, and Ni2p XPS (Fig. S5 and 7) reveal
only Ni, accompanied by NiOx formed by oxidation in air, fully covering the Cu/PTFE fibers.
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NO3 RR activity and kinetics. To investigate the electrocatalytic activity and kinetics of
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NO3 reduction, we tested each catalyst on rotating disk electrodes (RDEs). With the Cu50Ni50
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catalyst, we found an onset potential of ~0.25 V vs. RHE (pH = 14) for NO3 reduction (Fig.
2a). The current density then increased sharply to its transport-limited ceiling of ~170 mA cm-2
(according to equation 1 in Supporting information) at 100 rpm in 1 M KOH + 100 mM KNO3
(pH = 14) electrolyte. On the pure Cu catalysts, we found that a much more negative cathodic
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potential was required to reach this same NO3 -transport-limited current: the current density
was only 100 mA cm-2 at -0.2 V vs. RHE which was only 60% of that obtained by operating
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the Cu50Ni50 catalyst at the same potential. The pure Ni catalyst is almost inactive for NO3
reduction (Fig. S8a and b).
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The current density, normalized to the electrochemically active surface area (ECSA),
increases exponentially to ~1 mA cm-2 along with the cathodic potential for the case of Cu50Ni50.
In contrast, an evident multi-electron transfer process (Fig. 2a and b) was seen on the pure Cu
catalyst. Cu50Ni50 catalysts exhibited a 40% lower ECSA, determined by its double-layer
capacitance, compared to pure Cu (Fig. S9a-c). This translated to a 6-fold increase in ECSA-
normalized current density for Cu50Ni50, compared to pure Cu at 0 V vs. RHE (Fig. 2b). The
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intrinsic NO3 RR activity was significantly improved using the CuNi alloy systems.
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To gain insight into the kinetics, we plotted Koutecký–Levich (K-L) curves for NO3
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reduction on the Cu50Ni50 and pure Cu catalysts (Fig. 2c) using their current density vs.
potential (j-V) profiles in 1 M KOH + 10 mM KNO3 (pH = 14) electrolyte (Fig. S10 and S8c).
K-L analysis revealed a four-electron-transfer process for NH3 production on both the Cu50Ni50
and Cu catalysts. The kinetic current density obtained using the Cu50Ni50 catalyst was
calculated from the intercept of the K-L plot and was 220 mA cm-2 at -0.25 V vs. RHE, 2x
higher than in the case of pure Cu controls (Table S2).
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We investigated the E1/2 of NO3 RR on catalysts with different Cu:Ni ratios in 1 M KOH
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+ 10 mM KNO3 electrolyte. On Cu30Ni70, Cu50Ni50, Cu80Ni20 alloys, we found increasing NO3
RR E1/2 compared to the pure Cu catalyst. For instance, at 100 rpm, Cu50Ni50 catalyst exhibited
the highest E1/2 of 0.08 V vs. RHE among all catalysts, while an E1/2 of -0.045 V vs. RHE was
seen in the case of pure Cu (Fig. S11). The improvement in E1/2 further increased to ~120 mV
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when catalysts were tested in 100 mM KNO3 at the same rotating rate (Fig. 2a). For all NO3
concentrations, Cu50Ni50 catalysts perform better than the pure Cu as evidenced by the
upshifting of E1/2 and the reduced overpotential required for the same current density (Fig. 2d
and S7d).
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NO3 RR selectivity. We investigated NH3 selectivity using catalysts deposited on
Cu/PTFE supports, e.g. the Cu50Ni50 catalyst on PTFE (labeled Cu50Ni50/PTFE), in a flow
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electrolyzer.29 We quantified the NH3 product concentration as a function of a range of NO3
concentrations using an indophenol blue method (Fig. S12). To confirm that the NH3 produced
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indeed comes from NO3 reduction, NO3 electroreduction was performed using the same
catalyst (Fig. S13).
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We achieved a 99 ± 1% Faradaic efficiency (FE) for NH3 at ~-0.15 V vs. RHE on the
Cu50Ni50/PTFE catalyst in 1 M KOH + 100 mM KNO3 electrolyte (Fig. 2e and Table S3). The
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peak FE for NH3, on Cu50Ni50/PTFE, shifted to a 50 mV lower overpotential compared to that
of the pure Cu/PTFE (Fig. S14). The corresponding current density using Cu50Ni50/PTFE was
more than 1.3 times higher than that obtained using pure Cu controls (Table. S5). We checked
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for catalyst reconstruction following NO3 reduction after a 2-hour NO3 RR operation in 100
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pure Cu/PTFE (Fig. S15).
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Alloying with Ni increased the NH3 FE at low overpotentials (potentials > -0.1 V vs. RHE)
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in different NO3 concentrations. Cu50Ni50 catalysts enhanced the NH3 FE by over 20% at ~0
V vs. RHE compared to pure Cu (Fig. 2f, Table S4 and 5). Specifically, the highest NH3 FE is
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65 ± 3%, 84 ± 2% and 93 ± 2% in 1, 2, and 10 mM NO3 conditions at pH = 14, respectively.
In contrast, pure Cu is only able to attain a FE of 42 ± 3%, 59 ± 3% and 87 ± 3% at the same
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NO3 concentrations.
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We achieved a peak NH3 half-cell energy efficiency (EE) of 40% using Cu50Ni50/PTFE at
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50 mA cm-2 in 100 mM NO3 (Fig. 2g). This corresponds to a full-cell EE of 31% for this
catalyst at 50 mA cm-2 in the same electrolyte (Fig. S16), which is 1.3-fold improved compared
to the case of the pure Cu catalyst. A 31% NH3 full-cell EE was also obtained using
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Cu50Ni50/PTFE at 2 mA cm-2 in 2 mM NO3 (Fig. S16). This is ~6 times greater than that of the
pure Cu catalyst under the same condition.
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By studying both the NH3 FE on Cu80Ni20 and Cu30Ni70 catalysts in 1 mM NO3 , we found
that the Cu50Ni50 catalyst was the most active and selective catalyst (Fig. S17 and 18). The NH3
FE at -0.06 V vs. RHE was 58 ± 2%. Cu80Ni20 catalysts produced NH3 with a similar FE of 51
± 2%. Introducing 70% Ni into Cu caused a sharp decrease in NH3 FE from ~58% to ~31%.
Depositing pure Ni largely blocked the Cu sites underneath and further reduced the NH3 FE to
11 ± 1%.
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We implemented the Cu50Ni50 catalyst on a 3D porous electrode by depositing the
Cu50Ni50 catalyst onto a Cu foam with a pore size of ~200 μm (Fig. S19). We achieved, as a
result, a 90 mA cm-2 current density at -0.1 V vs. RHE, which is two-fold higher than the
Cu50Ni50/PTFE electrode in 1 M KOH + 100 M KNO3 (Fig. 4h). The NH3 FE was steady at
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~95% over 12 h of NO3 RR and at a 39% NH3 half-cell energy efficiency (EE). 15% of the
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input NO3 was converted into NH3 in a single pass (Fig. 4h and Table S7).
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Electronic structure studies. To shed light on the electronic structure of the CuNi alloy
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catalysts under NO3 RR conditions, we turned to operando hXAS.30 The Cu50Ni50 catalyst in 1
M KOH + 10 mM KNO3 electrolyte at a series of applied potentials exhibited pure metallic
features in both Ni and Cu K-edge spectra under steady-state operation conditions (Fig. 3a-d,
Fig. S20). We then calculated the operando coordination numbers (CNs) of Ni at different
potentials. The CNs of metal-metal bonds stayed above 11.5 at all potentials (Table S6 and 7).
This result suggests that, in our work, there is unlikely to be a prominent role for subsurface
oxygen species as previously reported in related catalysts under distinct electrochemical (acidic)
conditions.31
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Since the adsorption energy of intermediates is strongly correlated with the d-band center
position of catalysts,32 we performed UPS studies for the pure Cu and the CuNi alloys (Fig.
3e). The pure Cu catalyst exhibited a d-band center location of -2.84 eV (E-EF, Fermi level) on
the background-corrected spectrum. Increasing the Ni composition in the alloys causes an
upshift of the d-band center towards the Fermi level by 0.14, 0.28 and 0.32 eV for Cu80Ni20,
Cu50Ni50 and Cu30Ni70, respectively. These results were in agreement with the XPS results,
wherein we observed the 2p electron redistribution which leads to a positive shift of the Cu3d
band towards the Fermi level. This indicates decreasing anti-bonding occupation and stronger
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adsorbate bonding,32 which means that alloying Ni with Cu greatly enhances the adsorption
energies of intermediate species.
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Taken together, the activity, selectivity and d-band center positions, allow us to reason
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that enhanced NO3 RR intermediate adsorption, arising due to the shifted d-band center
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position, improved the NO3 RR activity and selectivity on CuNi alloys. However, introducing
an excess of Ni, i.e. the Cu30Ni70 catalyst, reaction intermediates too strongly, which lowers
the activity and selectivity.
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DFT studies. We sought to investigate, using DFT, the relationship between intermediate
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adsorption and the NO3 RR activity of different CuNi catalysts (Fig. 4a). The stability of CuNi
alloy systems were screened via doping Ni into Cu at various layers and distributions (Fig. S21,
22, and Table S8). Thermodynamics first force Ni atoms to replace the subsurface Cu when
the Ni:Cu ratio is less than 1:1. The substitution takes place on Cu surfaces with a further
increase in the ratio (Fig. S22).
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The electrochemical reaction NO3 + 6H2O + 8e- → NH3 + 9OH- was represented by a
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series of deoxidation reactions: *NO3 → *NO2 → *NO → *N followed by hydrogenation
reactions of *N → *NH → *NH2 → *NH3 according to a previous report.33 With the stable CuNi
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structures we built, we took different adsorbed orientations of intermediates on all possible
active sites into account (Fig. S23 and 24). For all intermediates, the most stable adsorption
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configurations (Fig. 4a) with the lowest total energies were employed to illustrate the NO3 RR
pathway, and hence assess the activities of different catalysts.
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A full NO3 RR pathway in 1 M KOH (pH = 14), including the deoxidation/hydrogenation
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reactions and intermediates,33 was then calculated (Fig. 4a). On pure Cu, the first NO3
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adsorption step is the potential-dependent step (PDS) of which the maximum reaction free
energy is 0.40 eV at -0.14 V vs. standard hydrogen electrode (SHE). Introducing Ni atoms
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moves the PDS from NO3 adsorption to the hydrogenation of the *NH2 intermediate (*NH2 +
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H2O + e- → *NH3 + OH-, Figure 4b), as the result of enhanced adsorption caused by the
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upshifted d-band center (Fig. 3e). A volcano-type relationship between the *NO3 adsorption
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energy and the NO3 RR experimental overpotentials was seen (Fig. 4c). By increasing Ni
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concentration in CuNi alloys, a stronger adsorption of *NO3 on the surface further modifies
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the energetics of NO3 RR. However, on Cu30Ni70 alloy and pure Ni, the reaction free energy
for *NH2 hydrogenation increases to -0.39 and -0.34 eV at -0.14 V vs. SHE, as high Ni fractions
lead to *NH2 intermediate adsorptions exceeding the optimal values. This fact, along with a
possible decrease in the number of Cu sites (Fig. S23 and 24), works against the formation of
*NH3 and the selectivity toward NH3 decreases as a result.
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Conclusions
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This work presents the relationship between intermediate adsorption energies and NO3
RR activity on CuNi catalysts. By replacing 50% Cu with Ni, we achieved significantly
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improved NO3 RR-to-NH3 performance. This includes a 0.12 V upshift in the half-wave
potential, a 0.2 V lower overpotential required to achieve the optimal NH3 FE, and a 6-fold
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increase in NO3 RR activity on Cu50Ni50 alloy catalysts compared to pure Cu at 0 V vs. RHE
in alkaline conditions (pH = 14). The electronic structure studies revealed an upshifting of the
d-band center toward the Fermi level, a feature that enhances intermediate adsorption energies.
This relationship was then validated by our DFT calculations, wherein we found that
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introducing Ni atoms moves the PDS from NO3 adsorption to *NH2 hydrogenation due to the
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enhanced adsorption energy of NO3 on the CuNi surface, and as a result, lowers the
overpotential. Our work demonstrates the effect of the d-band center positions and the induced
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adsorption properties on the NO3 RR activity and selectivity. This work highlights a promising
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route to design catalysts for selective NO3 RR to NH3.
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Acknowledgements
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This work was financially supported by the Ontario Research Fund: Research Excellence
Program, the Natural Sciences and Engineering Research Council (NSERC) of Canada, and
the CIFAR Bio-Inspired Solar Energy program. This research used synchrotron resources of
the Advanced Photon Source (APS), an Office of Science User Facility operated for the U.S.
Department of Energy (DOE) Office of Science by Argonne National Laboratory, and was
supported by the U.S. DOE under Contract No. DE-AC02-06CH11357, and the Canadian Light
Source and its funding partners. All DFT computations were performed on the IBM
BlueGene/Q supercomputer with support from the Southern Ontario Smart Computing
Innovation Platform (SOSCIP) and Niagara supercomputer at the SciNet HPC Consortium.
SOSCIP is funded by the Federal Economic Development Agency of Southern Ontario, the
Province of Ontario, IBM Canada Ltd., Ontario Centres of Excellence, Mitacs and 15 Ontario
academic member institutions. SciNet is funded by: the Canada Foundation for Innovation; the
Government of Ontario; Ontario Research Fund - Research Excellence; and the University of
Toronto. We acknowledge the Toronto Nanofabrication Centre (TNFC) and the Ontario Centre
for the Characterization of Advanced Materials (OCCAM) for sample preparation and
characterization facilities. The authors thank Dr. T. P. Wu, and L. Ma for technical support at
9BM beamline of APS. D.S. acknowledges the NSERC E.W.R Steacie Memorial Fellowship.
J.L. acknowledges the Banting postdoctoral fellowship from Govt. of Canada.
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Supplementary materials
Materials and Methods
Figs. S1 to S24
Tables S1 to S8
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Competing financial interests: The authors declare no competing financial interests.
Data and materials availability
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All data are reported in the main text and supplementary materials.
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Figures
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Figure 1. Materials characterization of copper-nickel alloy catalysts. (a, b) Representative
SEM and HRTEM images of the Cu50Ni50 catalyst. (c, d) Representative SEM and HRTEM
images of the pure Cu catalyst. The scale bars are 200 nm in a and c, and 10 nm in b and d.
(e-h) The STEM image and EELS mapping analysis of the Cu50Ni50 catalyst. The scale bars
are 100 nm. (i, j) XRD patterns and XPS Cu2p spectra of catalysts with different Cu:Ni ratios.
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Figure 2. Nitrate-to-ammonia electroreduction performance. (a) j-V plots of nitrate
reduction (80% iR corrected) on the Cu50Ni50, pure Cu and pure Ni RDE at 100 rpm in 1 M
KOH + 0.1 M KNO3 electrolyte. (b) ECSA-normalized current densities. (c) Koutecký–Levich
plots of nitrate reduction on Cu50Ni50 and Cu at -0.25 V vs. RHE in 1 M KOH + 10 mM KNO3
electrolyte. (d) j-V plots of nitrate reduction (80% iR corrected) on the Cu50Ni50 RDE at 400
rpm. (e) Nitrate-to-ammonia Faradaic efficiency on the Cu50Ni50/PTFE catalyst in different
nitrate concentrations. (f) Comparison of the highest NH3 Faradaic efficiency on the
Cu50Ni50/PTFE and pure Cu/PTFE catalysts in different nitrate concentrations. (g) Comparison
of the cathodic (half-cell) NH3 energy efficiency (EE) obtained using the Cu50Ni50/PTFE and
pure Cu/PTFE catalysts. (h) Stability and single-pass conversion test of nitrate reduction at -
0.1 V vs. RHE using a Cu50Ni50/Cu foam catalyst.
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Figure 3. Electronic structure. (a) Operando Cu K-edge hXAS spectra of the Cu50Ni50
catalyst at different applied potentials. (b) Fourier-transformed operando Cu K-edge hXAS
spectra of the Cu50Ni50 catalyst at different applied potentials. (c) Operando Ni K-edge hXAS
spectra of the Cu50Ni50 catalyst at different applied potentials. (d) Fourier-transformed
operando Ni K-edge hXAS spectra of the Cu50Ni50 catalyst at different applied potentials. (e)
The UPS spectra and d-band center positions of pure Cu catalysts and the CuNi alloys.
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Figure 4. DFT calculations. (a) Reaction free energies for different intermediates on CuNi
surface. (b). Hydrogenation reaction of *NH2 (*NH2 + H2O + e- → *NH3 +OH-) on Cu30Ni70
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surface. (c) The volcano-type relationship between experimental overpotentials of NO3 RR at
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5 mA cm-2 in 10 mM KNO3 and adsorption energies of *NO3 on all CuNi alloys. Red, pink,
blue, grey and orange spheres correspond to oxygen, hydrogen, nitrogen, nickel and copper
atoms, respectively.
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Figure 1. Materials characterization of copper-nickel alloy catalysts. (a, b) Representative SEM and HRTEM
images of the Cu Ni catalyst. (c, d) Representative SEM and HRTEM images of the pure Cu catalyst. The
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scale bars are 200 nm in a and c, and 10 nm in b and d. (e-h) The STEM image and EELS mapping analysis
of the Cu Ni catalyst. The scale bars are 100 nm. (i, j) XRD patterns and XPS Cu2p spectra of catalysts
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with different Cu:Ni ratios.
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Figure 2. Nitrate-to-ammonia electroreduction performance. (a) j-V plots of nitrate reduction (80% iR
corrected) on the Cu Ni , pure Cu and pure Ni RDE at 100 rpm in 1 M KOH + 0.1 M KNO electrolyte. (b)
50 50
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ECSA-normalized current densities. (c) Koutecký–Levich plots of nitrate reduction on Cu50Ni50 and Cu at -
0.25 V vs. RHE in 1 M KOH + 10 mM KNO electrolyte. (d) plots of nitrate reduction (80% iR corrected)
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j-V
on the Cu Ni RDE at 400 rpm. (e) Nitrate-to-ammonia Faradaic efficiency on the Cu50Ni50/PTFE catalyst
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in different nitrate concentrations. (f) Comparison of the highest NH Faradaic efficiency on the
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Cu Ni /PTFE and pure Cu/PTFE catalysts in different nitrate concentrations. (g) Comparison of the
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cathodic (half-cell) NH energy efficiency (EE) obtained using the Cu Ni /PTFE and pure Cu/PTFE
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catalysts. (h) Stability and single-pass conversion test of nitrate reduction at -0.1 V vs. RHE using a
Cu Ni /Cu foam catalyst.
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Figure 3. Electronic structure. (a) Operando Cu K-edge hXAS spectra of the Cu Ni catalyst at different
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applied potentials. (b) Fourier-transformed operando Cu K-edge hXAS spectra of the Cu Ni catalyst at
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different applied potentials. (c) Operando Ni K-edge hXAS spectra of the Cu Ni catalyst at different
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applied potentials. (d) Fourier-transformed operando Ni K-edge hXAS spectra of the Cu Ni catalyst at
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different applied potentials. (e) The UPS spectra and d-band center positions of pure Cu catalysts and the
CuNi alloys.
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Figure 4. DFT calculations. (a) Reaction free energies for different intermediates on CuNi surface. (b).
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Hydrogenation reaction of *NH (*NH + H O + e → *NH +OH ) on Cu Ni surface. (c) The volcano-
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-
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type relationship between experimental overpotentials of NO RR at 5 mA cm in 10 mM KNO and
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adsorption energies of *NO on all CuNi alloys. Red, pink, blue, grey and orange spheres correspond to
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oxygen, hydrogen, nitrogen, nickel and copper atoms, respectively.
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