Monitoring surface transformations of metal carbodiimide water oxidation catalysts by operando XAS and Raman spectroscopy

Article abstract of DOI:10.1039/c8dt01587b

Transition metal carbodiimides MNCN (M = Co, Ni, Co_0.9Ni_0.1, Mn and Cu), were studied by simultaneous operando Raman and X-ray absorption spectroscopy (XAS) with focus on surface oxide detection during electrocatalytic water oxidation. As a proof of concept, easily modifiable screen-printed electrodes were used in this unified operando synchrotron setup for a trade-off between convenience of electrochemical anodization and spectroscopic data acquisition. Monitoring of chemical and structural transformations at the electrode surface during initial anodic electrode polarization shows stability for MNCN with M = Co, Ni, Co_0.9Ni_0.1 and Mn. While MnNCN is inactive, CoNCN emerges as the most active representative of the series. CuNCN displays pronounced side reactions and the formation of a surface copper oxide layer leading to lower current density attributed to water oxidation, as evident from an irreversible variation of the CuNCN redox behaviour in rotating ring-disc voltammetry. Furthermore, the accompanying structural and vibrational spectroscopy properties of the different MNCN compounds were explored with complementary ex situ analytical methods.

Full text of DOI:10.1039/c8dt01587b

Dalton
Transactions
View Article Online
View Journal
PAPER
Monitoring surface transformations of metal
carbodiimide water oxidation catalysts by
Cite this: DOI: 10.1039/c8dt01587b
operando XAS and Raman spectroscopy†
Rafael J. Müller,
Marcella Iannuzzi and Greta R. Patzke
Jinggang Lan,
Karla Lienau, René Moré, C. A. Triana,
*
Transition metal carbodiimides MNCN (M = Co, Ni, Co0.9Ni0.1, Mn and Cu), were studied by simultaneous
operando Raman and X-ray absorption spectroscopy (XAS) with focus on surface oxide detection during
electrocatalytic water oxidation. As a proof of concept, easily modifiable screen-printed electrodes were
used in this unified operando synchrotron setup for a trade-off between convenience of electrochemical
anodization and spectroscopic data acquisition. Monitoring of chemical and structural transformations at
the electrode surface during initial anodic electrode polarization shows stability for MNCN with M = Co,
Ni, Co0.9Ni0.1 and Mn. While MnNCN is inactive, CoNCN emerges as the most active representative of the
series. CuNCN displays pronounced side reactions and the formation of a surface copper oxide layer
leading to lower current density attributed to water oxidation, as evident from an irreversible variation of
the CuNCN redox behaviour in rotating ring-disc voltammetry. Furthermore, the accompanying structural
and vibrational spectroscopy properties of the different MNCN compounds were explored with comp-
lementary ex situ analytical methods.
Received 20th April 2018,
Accepted 26th June 2018
DOI: 10.1039/c8dt01587b
rsc.li/dalton
In the ongoing search for highly efficient and robust water
oxidation catalysts (WOCs), earth-abundant materials have
1. Introduction
The search for sustainable, storable and scalable energy emerged as affordable and versatile targets for scalable hetero-
sources is among the most pressing current research issues,1 geneous applications,8 including oxides/hydroxides,9 perovs-
and the available portfolio of strategies for harvesting renew- kites,10 phosphates/phosphines11,12 and polyoxometalates.13
able energy is constantly developed further.2 Artificial photo- Among these materials types, cobalt based catalysts exhibit
synthesis and water splitting by photovoltaics-electrolysis offer remarkable activity,14 but their efficiency-to-stability perform-
direct alternative routes for clean hydrogen fuel production, ance strongly depends on the electrode surface properties under
and its conversion into convenient storage options.3 The turnover conditions, where oxidation,15 intercalation,16 electro-
efficiency of both approaches is presently determined by the deposition,17 amorphization,18,19 dissolution20 and leaching21
thermodynamically and kinetically challenging oxygen evol- phenomena can occur.22 Understanding those surface trans-
ution reaction (OER) as a proton coupled four-electron multi- formations is a challenging task that requires pioneering spec-
step transfer process.4 Understanding the precise mechanisms troscopic techniques and operando monitoring. The latter tech-
behind the bottleneck5 of energetically uphill charge accumu- niques offer direct insight into the processes of electrode acti-
lation around oxygen evolving catalytic centres is the key to vation or passivation.23 Raman24 and X-ray absorption spec-
their tailored design.6 Knowledge of the detailed structural troscopy (XAS)12,19,25 provide detailed information about cataly-
and chemical properties of catalytic materials is thus indis- tically active sites and their chemical environment even for
pensable,6 especially of catalyst surfaces and their coordi- monolayer-range26 surface transformations.
nation–interaction with water molecules throughout the cata-
Metal carbodiimides MNCN (M = Co and Ni) were recently
introduced as promising water oxidation catalysts.27 They
provide fundamental electronic and structural relations to pro-
minent oxide catalysts, such as octahedral units as present in
CoOx, while their oxygen-free compositions permit a clear dis-
tinction of the catalytic centres from the MNCN matrix.28
Therefore the all-nitrogen {MN6} coordination of the active tran-
sition metal centres in MNCN-based WOCs (Fig. 1) is in prin-
lytic cycle.7
University of Zurich, Department of Chemistry, Winterthurerstrasse 190,
CH-8057 Zurich, Switzerland. E-mail: greta.patzke@chem.uzh.ch
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8dt01587b
This journal is © The Royal Society of Chemistry 2018
Dalton Trans.

View Article Online
Paper
Dalton Transactions
amide with ammonium carbonate following reported proto-
cols.27,30,31 CuNCN and MnNCN were obtained directly
through dissolution of the respective metal chloride in molten
H2NCN at 75 °C and precipitation by addition of (NH4)2CO3.
Following the same procedure, Co and Ni first afforded
M(HNCN)2 precursors, which were subsequently decomposed
thermally in a LiCl/KCl eutectic mixture and N2 atmosphere at
400 °C to yield CoNCN and NiNCN (see ESI, section 1† for
details). Reference materials, such as Co3O4 and CuO, were
obtained commercially in analytical grade purity.
Fig. 1 Left: Representative crystal structure of CoNCN showing the
octahedral edge sharing {CoN6} coordination sheets of MNCN. Right:
Operando XAS and Raman setup at the ESRF BM01 Swiss Norwegian
Beamline: 1. XAS fluorescence detector; 2. Raman detector; 3. X-ray
source; 4. Raman source; 5. SPE connector; 6. X-ray path for
transmission.
2.2 Structural characterization
Structural phase characterization was carried out with powder
X-ray diffraction (PXRD) on a STOE STADI P diffractometer
operated in transmission mode with a Ge monochromator and
Mo-Kα1 radiation.
ciple a unique platform to study three key aspects of the water
oxidation process: (1) the photoelectrochemical potential of
different catalytic centres in the absence of an oxide lattice
environment, (2) the water oxidation mechanism with special
emphasis on the H2O and OH− coordination to isolated metal
centres in a stable non-oxide matrix, and (3) the surface con-
fined catalyst–oxygen interaction without background signals
from bulk oxides. This knowledge is essential for informed
WOC construction through linking key concepts of homo-
geneous and heterogeneous catalysis.6
Moreover, the MNCN series excels through substitutional
versatility on the M site for further improvement of the photo-
catalytic performance.27 However, complete loss of X-ray diffr-
action signals during Na+-ion intercalation reactions was
recently revealed for CuNCN electrodes.29 This raises the ques-
tion whether related processes can affect the performance of
the MNCN series at the lower potentials used for aqueous
electrocatalysis.
We here apply simultaneous operando Raman and X-ray
absorption spectroscopy to monitor surface-structure transform-
ations on easily modifiable screen-printed electrodes of metal
carbodiimides MNCN (M = Co, Co0.9Ni0.1, Ni, Mn and Cu) during
electrocatalytic water oxidation. This integrated synchrotron
setup offers a trade-off between fast and flexible electrochemical
anodization and operando data acquisition. This strategy is essen-
tial to check the MNCN series for reversible operation of their
active metal centres and for the formation of any active surface-
oxide layers. In the following, we demonstrate successful moni-
toring of structural transformations at MNCN electrode surfaces
during initial anodic electrode polarization together with an
assessment of catalytic stability. These results are complemented
further with experimental ex situ spectroscopy and analysis of
state-of-the-art vibrational phonon calculations.
2.3 Ex situ vibrational spectroscopy
Ex situ Fourier transform infrared spectroscopy (FT-IR) was
performed employing
a
Bruker-Vertex 70 spectrometer
equipped with an ATR diamond crystal. Ex situ Raman spectra
were recorded on a Renishaw Ramascope (Ar+ laser, 514 nm)
or an InVia Qontor (532, 633, 785 nm) for pristine powder
samples on quartz glass slides. All Raman spectra exhibited
basically identical vibrational modes, independent of the
applied excitation wavelength (see ESI, section 4.1† for
details).
2.4 Electrode modification
The screen-printed electrode (SPE) glassy carbon working area
was conveniently modified by dropcasting catalyst-Nafion ink
with micropipettes. The ink was prepared by sonicating a
suspension of 5.0
0.2 mg MNCN (mortar ground) and
42.2 µL Nafion (5% wt sol.) in 320 µL EtOH for 10 min. The
final ink contained 5.4 mg mL−1 Nafion and 13.8 mg mL−1
catalyst. An electrode loading series with 1 µL, 5 µL and 10 µL
ink was prepared resulting in partial, full and excess coverage
of the SPE working electrode area. Nafion is commonly used
as a binder for WOCs and considered oxidatively inert,32 so
that chemical stability is warranted for the moderate current
densities (<10 mA cm−2) and overpotentials (<700 mV) applied
in the present study. A chronoamperometric analysis reprodu-
cing the potential steps as applied during operando conditions
is shown in the ESI, section 2.2.†
2.5 RRDE voltammetry
RRDE experiments were performed by scanning the working
disc electrode potential from −0.3 to 1.2 V at a rotation speed
of 1600 rpm and constant ring electrode potential of −0.8 V,
which is in the mass transfer limited region of oxygen
reduction on platinum (see ESI, section 3† for details).
2. Experimental
2.6 Electrochemical procedures
2.1 Synthetic methods
Electrochemical standard characterization measurements were
The carbodiimide series MNCN (M = Co, Co0.9Ni0.1, Ni, Mn, performed in a three-electrode setup connected to a Metrohm
Cu) was synthesized from metal chlorides in dihydrogencyan- Autolab N203 potentiostat using a Ag/AgCl (3 M KCl) reference
Dalton Trans.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Dalton Transactions
Paper
electrode (RE) and a glassy carbon rod counter electrode (CE).
Working electrodes (WE) were either a glassy carbon disc (r =
1 mm), carbon disc/platinum ring RRDE or a Dropsens
(DRP-110) SPE (see ESI, section 2 and 3† for details). The elec-
trolyte was 0.1 M KOH (Aldrich 99.99%) in ultrapure water (R >
18 MΩ). No compensation for electrolyte resistance (500 Ω by
EIS high frequency limit) was applied. Under alkaline con-
ditions no degradation was detected for the MNCN com-
pounds. ICP-MS analyses of the supernatant showed a metal
content after 20 h of catalyst aging in 0.1 and 1.0 M KOH solu-
tion corresponding to 0.02% of the initial mass. We thus con-
sider leaching from catalysts negligible, given that samples
were immobilized in Nafion layers during an approx. 10-times
shorter reaction timescale for the operando experiments (see
ESI, section 4.3† for details).
Fig. 2 PXRD patterns of the carbodiimide series MNCN (M = Co,
Co0.9Ni0.1, Ni, Mn, Cu).
2.7 Operando monitoring setup
Operando Raman and XAS experiments were carried out at the
ESRF BM01 Swiss Norwegian Beamline. An electrode connec-
tor was placed in front of the beam path to hold the SPE
surface tilted by 15°–20° towards the X-ray beam (Fig. 1). The
position of the Vortex fluorescence detector was above the SPE
(normal to the X-ray beam, 70°–75° to the SPE). Raman spectra
were recorded using a Renishaw system equipped with a
785 nm laser that was focused on the SPE surface using an IR
camera. The penetration depth for X-rays around the metal
K-edge for XAS extends to 60–100 μm. For Raman spectroscopy
measurements, the penetration depth using an excitation
wavelength of 785 nm extends to 4.6–5.8 0.5 μm. Thin oxide
layer formation is expected to occur largely at the surface, so
that the lower penetration depth of Raman spectroscopy is in
principle sufficiently surface sensitive to detect the formation
of such oxide layers.
the reference patterns: CoNCN [P63/mmc, ICDD 04-015-1039],
NiNCN [P63/mmc, ICDD 04-015-1038], MnNCN [R3m, ICDD
04-011-7543], and CuNCN [Cmcm, ICDD 04-012-5725]. No
additional crystalline phases were found.
Ex situ FT-IR spectra of MNCN (M = Mn, Co, Ni; Fig. 3) show
the asymmetric stretching mode νs of the NCN matrix at
2052–2083 cm−1 and the doubly degenerated deformation
mode δ at 647–658 cm−1. The FT-IR spectrum of CuNCN shows
additional and further shifted signals due to deviations from
linear geometry of NCN, which lowers the symmetry and allows
additional vibrations. The symmetric stretching and defor-
mation modes of the NCN moiety were reported to occur
around 2000 cm−1 and 666 cm−1, respectively.9 Generally, over-
lapping peaks of side products occur in the 1100–1700 cm−1
range which can be attributed to cyanamide polymerization for
MnNCN and CuNCN and melamine polycondensation for
CoNCN and NiNCN. The very weak signals above 3000 cm−1 can
2.8 Operando spectroscopy at constant potential
amperometry
For operando spectroscopy at constant potential, the SPE was
dropwise covered with KOH electrolyte and chronoampero-
metry experiments were conducted by a stepwise increase of
the anodic potential in the range 0.0–1.2 V. During each 0.1 V
step of 10 min five XAS spectra were measured and Raman
spectra were continuously accumulated. The Raman shift
range during the operando experiments was limited to
100–1000 cm−1 to allow multiple spectra accumulations.
2.9 Ex situ X-ray absorption spectroscopy
Pristine catalyst powder and reference compounds were mixed
with cellulose to prepare pellets for ex situ XAS measurements
in transmission mode.
3. Results and discussion
3.1 Ex situ characterization
Evaluation of the PXRD patterns (Fig. 2) shows that the synthe-
Fig. 3 Ex situ FT-IR spectra of the pristine carbodiimides MNCN
tized MNCN products agree with the crystallographic phases of (M = Co, Co0.9Ni0.1, Ni, Mn, Cu).
This journal is © The Royal Society of Chemistry 2018
Dalton Trans.

View Article Online
Paper
Dalton Transactions
either result from the same side products or from combina- implemented phonon calculations (see ESI, section 4.4† for
tional NCN modes (νs + νas = 3203–3211 cm−1). Further discus- details). The calculated IR and power spectra show that all
sions on the purity of the MNCN series are provided in the ESI vibrational peaks in the range of 300–900 cm−1 correspond to
(section 4†). Most importantly, the main product peaks match vibrational modes arising from the CoNCN matrix. In contrast,
with the expected values and no oxide peaks were detected.
the peaks at 300, 345 and 620 cm−1 observed in the Raman
Ex situ Raman spectra (Fig. 4) agree well with the reported spectra of post-catalytic CuNCN do not appear in previously
data for CoNCN,27 showing the allowed symmetric NCN calculated phonon spectra of CuNCN,33 so that they indeed
stretching mode νs around 1370 cm−1. The peak at 1265 cm−1 most likely correspond to CuO oxide peaks (see Fig. 4). Further
may be assigned to the redshifted overtone of the bending comparisons of the ex situ Raman spectra of pristine, pre- and
mode (2δIR = 2 × 648 cm−1 = 1296 cm−1), which may occur with post-catalytic CuNCN and CoNCN SPEs with those of reference
increased intensity as a result of mixing of vibrational states oxides (see ESI Fig. S4†), shows that only the CuNCN post-cata-
(Fermi resonance).10 The signal of the NCN-bending occurs at lytic spectrum matches the expected CuO peaks.
higher wavenumbers compared to FT-IR (δRaman
=
Reproduction of the chronoamperometric experiment with
650–700 cm−1). Other characteristic Raman peaks of the 18O labelled water, followed by immediate analysis of the
MNCN series are located in the lattice vibration region (below sample, confirmed the transformation of CuNCN through
500 cm−1) at 400 cm−1 and 220 cm−1. Two broad graphitic clearly observable oxygen-related modes (see ESI Fig. S9a†). No
peaks at 1300 cm−1 and 1600 cm−1 are indicative for g-C3N4 in such peak shifts were observed for Raman analyses of CoNCN,
the pristine samples and overlap with the substrate carbon Co0.9Ni0.1NCN and NiNCN after electrocatalysis in 18O labelled
peaks at the same positions in post-catalytic SPE samples. The water, which further confirms the absence of oxides in these
asymmetric stretching mode νas appears at around catalysts (see ESI Fig. S9b–d†).
2000–2100 cm−1
.
The voltammetric response of SPE working electrodes
The post catalytic spectrum of CuNCN shows CuO peaks (Fig. 5) was characterized with a conventional electrochemical
and the disappearance of the peak at 400 cm−1. For the other cell setup using a GC-rod CE and Ag/AgCl RE. CoNCN and
MNCN samples only additional overlapping carbon back- Co0.9Ni0.1NCN are slightly more active than Co3O4 with the sig-
ground is present around 1500 cm−1 after the catalytic process. nificant difference that they show much less pronounced pre-
To verify the peak assignments and to emphasize the dis- waves that also contribute to the current density. NiNCN dis-
tinction between Co–O or Co–NCN vibrational modes, we have plays an intermediate performance, while CuNCN shows only
minor activity after reaching
a stable response. Finally,
MnNCN coating affords even lower current density than the
blank uncoated SPE. Tafel plots obtained from chronoampero-
metric analysis (see ESI, Fig. S2b†) of coated SPEs are not
expected to be as precisely representative as rotating disc elec-
trodes, because the membrane character of the layer, diffusion
Fig. 5 CV of SPEs with 2 µL catalyst ink loading measured in an electro-
chemical cell setup using GC-rod CE and Ag/AgCl RE (0.1 M KOH, scan
rate: 10 mV s−1). The inset shows LSV results of the catalytically active
Fig. 4 Pre- and post-catalytic Raman spectra of MNCN (pre: pristine
material, post: post operando SPE, a CuO spectrum is provided as refer-
ence for CuNCN).
SPEs with
1 µL catalyst ink loading (overpotential measured at
80 μA cm−2, maximum current density at 1.1 V vs. Ag/AgCl).
Dalton Trans.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Dalton Transactions
Paper
overpotentials and restricted convection are not accounted for. performance. When anodically increasing the applied poten-
Qualitatively, however, the results of chronoamperometric tial from 0.0 to 1.2 V, the Cu absorption K-edge in the normal-
potential steps are in line with the voltammetry and further ized spectra (Fig. 6b), derived from the edge-half-height step-
show that up to a current density of 1 mA cm−2 Co- and position and from the edge first derivative maximum, respect-
Ni-carbodiimides have lower Tafel slopes below 100 mV per ively, is shifted about 1.0 eV towards higher energy. However,
decade, while reaching a most likely diffusion limited region the magnitude of this shift may be due to a systematic drift
with a high Tafel slope above 300 mV dec−1 at higher current and cannot be explicitly interpreted as a change in the oxi-
densities, which is comparable to the blank and to the less dation state of Cu. Surface oxide formation is far more clearly
active CuNCN and MnNCN samples. Further chronoampero- revealed by the variation in the post-edge region at about
metric tests over extended periods as applied during operando 9008–9026 eV (highlighted region in Fig. 6a). Comparison of
studies confirmed these trends (Fig. S2c†). The feasibility of the pre- and post-catalytic XAS spectra with a CuO reference
using the SPE Ag-pseudoreference and CE was tested by slow indicates that the XAS signal changes significantly above 0.9 V
linear sweep voltammetry (LSV) at a scan rate of 1 mV s−1 to resemble that of the oxide. A rise of absorption intensity at
(inset in Fig. 5). A similar trend emerges, and the current 9015 eV is observed, which is the most pronounced spectral
density of CuNCN in this mode is increased by its unequili- difference of CuO compared to CuNCN. The white-line shift of
brated conditioning current. The Ag-pseudoreference was shifted about 2 eV (Fig. 6b), which is twice as high as the usually
roughly 0.09 V to lower potential versus the Ag/AgCl reference.
observed fluctuations of 1 eV, can be interpreted as additional
evidence for changes in the Cu surface coordination state. A
linear combination fit of the XANES region in the range of
8995–9050 eV (Fig. 6b, green symbols) affords clearer results:
operando XAS spectra were fitted with a CuO reference spec-
trum and with the initial electrode state at 0.0 V. Until 0.9 V
the fraction of CuO ranges below 20% and may well result
from background adjustments. In contrast, the CuO fraction
then rises sharply at 1.0 V and reaches up to 80% at 1.2 V, thus
significantly indicating CuO formation.
3.2 Operando characterization of the MNCN series
Operando XAS spectra of CuNCN are evaluated in Fig. 6 to
directly monitor the processes leading to lower electrocatalytic
The remaining MNCN (M = Mn, Co, Co0.9Ni0.1, Ni) com-
pounds did neither exhibit consistent post edge changes, nor
did their absorption edge features change significantly with
the applied anodic potential. Only fluctuating absorption edge
shifts (< 1 eV) were observed for the CoNCN and NiNCN cata-
lysts, which are within the noise level and the resolution limits
of XAS data acquisition (0.5 to 1.0 points per eV, see the
spectra in the ESI, section 5†).
Operando Raman spectra for CuNCN do not show the
expected oxide peaks at 300 (s), 350 (s) and 630 (w) cm−1 due
to low scattering intensity (see the spectra in the ESI, section
6†). However, post-catalytic ex situ measurements show pres-
ence of CuO (see Fig. 4).
Operando Raman spectra of CoNCN with significant peak
intensity are shown in Fig. 7. During anodization the Raman
peak intensity at 220 cm−1 slightly decreases, going hand in
hand with higher background towards lower wavenumbers and
declining overall scattering intensity. Except for the appearance
of a shoulder of the phonon peak at 400 cm−1, the other
CoNCN bands remain unchanged within the available resolution.
All in all, no peaks indicative of metal oxide formation were
detected by operando and ex situ Raman spectroscopy of
CoNCN, Co0.9Ni0.1NCN and MnNCN.
In order to further investigate the observed CuO formation
and to separate the oxygen evolution current of CuNCN from
other accompanying redox processes, rotating ring disc elec-
trode (RRDE) experiments were performed.
Cyclic voltammetry of CuNCN modified RRDEs clearly con-
firms that the overall current density response of initial anodic
polarization encompasses current contributions from side
Fig. 6 (a) XANES (operando, fluorescence mode) of CuNCN and pre-/
post catalytic spectra as reference. (b) CuNCN operando XANES descrip-
tors (whiteline peak and edge energy) vs. potential; right axes: current
and CuO fraction based on linear combination fit results of the XANES
regions of CuNCN and CuO.
This journal is © The Royal Society of Chemistry 2018
Dalton Trans.

View Article Online
Paper
Dalton Transactions
scan, indicating an increase of oxygen arriving at the ring.
This can be caused either by an increasing fraction of oxygen
generating current at the disc during the reverse scan or by a
delayed desorption of the formed oxygen from the disc catalyst
layer. The magnitude of this irreversible conditioning current
contribution is considerable and can easily be mistaken for
the onset of oxygen evolution, which is in fact detected at the
ring only above ca. 0.7 V. In contrast, the corrected ring
current of CoNCN coincides with the disc current almost per-
fectly and remains stable during the following cycles, indicat-
ing only small contributions of side reactions or non-faradaic
currents. During consecutive cycles of CuNCN the current
density between 0.4 and 0.8 V is significantly lower and
decreases until a steady state is reached after about 3 cycles
(Fig. 8b). Subtraction of the corrected ring current from the
disc current reveals a difference voltammogram that resembles
a pseudo reversible redox wave (anodic peak at 0.8 V) and is
attributed to the oxidative transformations of surface CuNCN.
At steady state the remaining disc/ring current difference is
below 2 mA cm−2 and reversible for both CoNCN and CuNCN.
This current difference may be attributed to surface polariz-
ation, adsorption and desorption or capacitive effects.
Fig. 7 Operando Raman spectra of CoNCN measured at different
applied voltages. No oxide related peaks occur.
reactions (Fig. 8b). The corrected oxygen reduction current at
the ring is substantially lower than the disc current. The ring
current of the cathodic scan is higher compared to the anodic
Furthermore, the lower catalytic water oxidation activity of
CuNCN compared to CoNCN is reflected in the respective
current densities of 10 mA cm−2 vs. 60 mA cm−2 at 1.2 V.
4. Conclusions
Operando Raman and XAS spectroscopy were applied to
monitor the surface transformations of metal carbodiimides
MNCN (M = Co, Ni, Co0.9Ni0.1, Mn and Cu) on screen-printed
electrodes (SPE) during electrocatalytic water oxidation.
Chemical and structural transformations of these electrodes
under the anodically applied potential could be detected with
the current setup through observed differences in the XAS
characteristics, as shown by the operando CuO formation on
CuNCN modified SPEs. Absence of oxide formation for the
CoNCN and NiNCN electrocatalysts as well as for MnNCN indi-
cates their higher stability under catalytic conditions.
The successful detection and identification of surface trans-
formations of catalyst loadings well below 1 mg on SPE using
operando X-ray absorption spectroscopy is a proof of concept
for new trade-offs between convenient and flexible sample
preparation and detailed analytical insight. The obtained
results are sensitive to various sample types and properties, as
evident from a series of Raman spectra acquisitions that
afforded satisfactory intensity only for the sufficiently well scat-
tering samples. Further optimization strategies are required in
such cases to specifically match the different analytical
requirements associated with XAS and Raman, respectively, for
low quantities of different catalyst types.
The present study paves the way to obtain fundamental
insight into concerted changes of the surface structural pro-
perties and oxidation states of electrocatalysts in a tuneable
and easily accessible electrode setup. Collecting comprehen-
Fig. 8 RRDE results for (a) CuNCN, (b) CoNCN: insets show a magnified
view of low current density. Dashed curves represent the second and
third scan cycle for CuNCN. Difference plots were gained by subtraction
of the collection efficiency corrected ring current from the disc current.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Dalton Transactions
Paper
sive information about surface and structural modifications
among entire catalyst series as a function of the applied poten-
tial is indispensable to systematically elucidate their common
mechanistic descriptors for heterogeneous catalysis. In the
long term, this strategy will allow to directly address open
questions about the basic design requests for optimal metal
nuclearities and coordination environments in cost-effective,
robust and efficient WOCs systems.
Vazquez, C. J. Carmalt, I. P. Parkin and J. R. Durrant, ACS
Catal., 2017, 7, 4896–4903.
6 J. Li, R. Güttinger, R. More, F. Song, W. Wan and
G. R. Patzke, Chem. Soc. Rev., 2017, 46, 6124–6147.
7 (a) P. W. Menezes, A. Indra, A. Bergmann, P. Chernev,
C. Walter, H. Dau, P. Strasser and M. Driess, J. Mater.
Chem. A, 2016, 4, 10014–10022; (b) A. M. Ullman,
C. N. Brodsky, N. Li, S.-L. Zheng and D. G. Nocera, J. Am.
Chem. Soc., 2016, 138, 4229–4236; (c) J. B. Gerken,
J. G. McAlpin, J. Y. C. Chen, M. L. Rigsby, W. H. Casey,
R. D. Britt and S. S. Stahl, J. Am. Chem. Soc., 2011, 133,
14431–14442.
8 (a) J. R. Galán-Mascarós, ChemElectroChem, 2015, 2, 37–50;
(b) B. M. Hunter, H. B. Gray and A. M. Müller, Chem. Rev.,
2016, 116, 14120–14136; (c) I. Roger, M. A. Shipman and
M. D. Symes, Nat. Rev. Chem., 2017, 1, 0003; (d) M. Bärtsch,
M. Sarnowska, O. Krysiak, C. Willa, C. Huber, L. Pillatsch,
S. Reinhard and M. Niederberger, ACS Omega, 2017, 2,
4531–4539.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Operando XAS and Raman experiments were performed at the
Swiss Norwegian Beamline BM01B of the European
Synchrotron Radiation Facility (ESRF), Grenoble, France. We
are grateful to Hermann Emerich (ESRF) for providing assist-
ance with beamline BM01B. Funding by the University
Research Priority Program “Solar Light to Chemical Energy
Conversion” (URPP LightChEC) of the University of Zurich and
by the Swiss National Science Foundation (Sinergia Grant No.
CRSII2_160801/1) is gratefully acknowledged. We thank Prof.
Dr Angelika Brückner and Dr Nils Rockstroh (Leibniz Institute
for Catalysis, Rostock/Germany) for reference Raman spec-
troscopy investigations.
9 S. Reinhard, F. Rechberger and M. Niederberger,
ChemPlusChem, 2016, 81, 935–940.
10 (a) J. Suntivich, K. J. May, H. A. Gasteiger,
J. B. Goodenough and Y. Shao-Horn, Science, 2011, 334,
1383–1385; (b) J. T. Mefford, X. Rong, A. M. Abakumov,
W. G. Hardin, S. Dai, A. M. Kolpak, K. P. Johnston and
K. J. Stevenson, Nat. Commun., 2016, 7, 11053.
11 (a) M. W. Kanan and D. G. Nocera, Science, 2008, 321,
1072–1075; (b) Y. Surendranath, M. W. Kanan and
D. G. Nocera, J. Am. Chem. Soc., 2010, 132, 16501–16509;
(c) H. Kim, J. Park, I. Park, K. Jin, S. E. Jerng, S. H. Kim,
K. T. Nam and K. Kang, Nat. Commun., 2015, 6, 8253.
12 M. W. Kanan, J. Yano, Y. Surendranath, M. Dinca,
V. K. Yachandra and D. G. Nocera, J. Am. Chem. Soc., 2010,
132, 13692–13701.
References
1 (a) X. Sala, I. Romero, M. Rodríguez, L. Escriche and
A. Llobet, Angew. Chem., Int. Ed., 2009, 48, 2842–2852;
(b) S. Chu and A. Majumdar, Nature, 2012, 488, 294–303; 13 (a) M. Blasco-Ahicart, J. Soriano-López, J. J. Carbó,
(c) S. Y. Tee, K. Y. Win, W. S. Teo, L.-D. Koh, S. Liu,
C. P. Teng and M.-Y. Han, Adv. Sci., 2017, 4, 1600337.
2 (a) H. Chen, T. N. Cong, W. Yang, C. Tan, Y. Li and Y. Ding,
Prog. Mater. Sci., 2009, 19, 291–312; (b) P. A. Owusu,
J. M. Poblet and J. R. Galán-Mascarós, Nat. Chem., 2018, 10,
24–30; (b) J. Soriano-López, D. G. Musaev, C. L. Hill,
J. R. Galán-Mascarós, J. J. Carbó and J. M. Poblet, J. Catal.,
2017, 350, 56–63.
S. Asumadu-Sarkodie and S. Dubey, Cogent Eng., 2016, 3, 14 (a) V. Artero, M. Chavarot-Kerlidou and M. Fontecave,
891.
Angew. Chem., Int. Ed., 2011, 50, 7238–7266; (b) J. Wang,
W. Cui, Q. Liu, Z. Xing, A. M. Asiri and X. Sun, Adv. Mater.,
2016, 28, 215–230; (c) H.-Y. Wang, J. Liu, J. Zhu, S. Styring,
S. Ott and A. Thapper, Phys. Chem. Chem. Phys., 2014, 16,
3661–3669; (d) H.-Y. Wang, E. Mijangos, S. Ott and
A. Thapper, Angew. Chem., Int. Ed., 2014, 53, 14499–14502.
3 (a) N. S. Lewis, Science, 2007, 315, 798; (b) H. Ahmad,
S. K. Kamarudin, L. J. Minggu and M. Kassim, Renewable
Sustainable Energy Rev., 2015, 43, 599–610; (c) C. Graves,
S. D. Ebbesen, M. Mogensen and K. S. Lackner, Renewable
Sustainable Energy Rev., 2011, 15, 1–23.
4 (a) H. Dau, C. Limberg, T. Reier, M. Risch, S. Roggan and 15 Y.-C. Liu, J. A. Koza and J. A. Switzer, Electrochim. Acta,
P. Strasser, ChemCatChem, 2010, 2, 724–761; (b) M. Tahir, 2014, 140, 359–365.
L. Pan, F. Idrees, X. Zhang, L. Wang, J.-J. Zou and 16 X. Li, X. Hao, Z. Wang, A. Abudula and G. Guan, J. Power
Z. L. Wang, Nano Energy, 2017, 37, 136–157. Sources, 2017, 347, 193–200.
5 (a) H. Inoue, T. Shimada, Y. Kou, Y. Nabetani, D. Masui, 17 (a) Y. Surendranath, D. A. Lutterman, Y. Liu and
S. Takagi and H. Tachibana, ChemSusChem, 2011, 4, 173–
179; (b) N. Yatom, Y. Elbaz, S. Navon and M. Caspary
Toroker, Phys. Chem. Chem. Phys., 2017, 19, 17278–17286;
D. G. Nocera, J. Am. Chem. Soc., 2012, 134, 6326–6336;
(b) D. A. Lutterman, Y. Surendranath and D. G. Nocera,
J. Am. Chem. Soc., 2009, 131, 3838–3839.
(c) A. Kafizas, Y. Ma, E. Pastor, S. R. Pendlebury, C. Mesa, 18 (a) A. Indra, P. W. Menezes, N. R. Sahraie, A. Bergmann,
L. Francàs, F. Le Formal, N. Noor, M. Ling, C. Sotelo-
C. Das, M. Tallarida, D. Schmeisser, P. Strasser and
This journal is © The Royal Society of Chemistry 2018
Dalton Trans.

View Article Online
Paper
Dalton Transactions
M. Driess, J. Am. Chem. Soc., 2014, 136, 17530–17536; 27 D. Ressnig, M. Shalom, J. Patscheider, R. Moré,
(b) X. Leng, Q. Zeng, K.-H. Wu, I. R. Gentle and
D.-W. Wang, RSC Adv., 2015, 5, 27823–27828.
F. Evangelisti, M. Antonietti and G. R. Patzke, J. Mater.
Chem. A, 2015, 3, 5072–5082.
19 A. Bergmann, E. Martinez-Moreno, D. Teschner, 28 (a) X. Liu, M. Krott, P. Müller, C. Hu, H. Lueken and
P. Chernev, M. Gliech, J. F. de Araújo, T. Reier, H. Dau and
P. Strasser, Nat. Commun., 2015, 6, 8625.
R. Dronskowski, Inorg. Chem., 2005, 44, 3001–3003;
(b) X. Liu, R. Dronskowski, R. K. Kremer, M. Ahrens, C. Lee
and M.-H. Whangbo, J. Phys. Chem. C, 2008, 112, 11013–
11017; (c) M. Krott, X. Liu, B. P. T. Fokwa, M. Speldrich,
H. Lueken and R. Dronskowski, Inorg. Chem., 2007, 46,
2204–2207; (d) M. Krott, A. Houben, P. Müller, W. Schweika
and R. Dronskowski, Phys. Rev. B: Condens. Matter Mater.
Phys., 2009, 80, 24117; (e) T. D. Boyko, R. J. Green,
R. Dronskowski and A. Moewes, J. Phys. Chem. C, 2013,
117, 12754–12761; (f) R. Nelson, P. M. Konze and
R. Dronskowski, J. Phys. Chem. A, 2017, 121, 7778–7786;
(g) D. Koziej, F. Krumeich, R. Nesper and M. Niederberger,
J. Mater. Chem., 2009, 19, 5122.
20 S. Zou, M. S. Burke, M. G. Kast, J. Fan, N. Danilovic and
S. W. Boettcher, Chem. Mater., 2015, 27, 8011–8020.
21 (a) S. Jony, C. D. Roy, J. Priyajit and P. Amit, Chemistry,
2017, 23, 12519–12526; (b) B. M. Hunter, N. B. Thompson,
A. M. Müller, G. R. Rossman, M. G. Hill, J. R. Winkler and
H. B. Gray, Joule, 2018, 2, 747–763.
22 M. S. Burke, M. G. Kast, L. Trotochaud, A. M. Smith and
S. W. Boettcher, J. Am. Chem. Soc., 2015, 137,
3638–3648.
23 C. H. M. van Oversteeg, H. Q. Doan, F. M. F. de Groot and
T. Cuk, Chem. Soc. Rev., 2017, 46, 102–125.
24 (a) Y. Teng, W. Liu, M. Lan, S. Ma and C. Ma, Anal. Lett., 29 A.
2015, 49, 299–306; (b) Y. Teng, G. Ding, W. Liu, J. Liu,
Y. Nie and P. Li, Surf. Interface Anal., 2016, 48, 990–994.
Eguía-Barrio,
E.
Castillo-Martínez,
X.
Liu,
R. Dronskowski, M. Armand and T. Rojo, J. Mater. Chem. A,
2016, 4, 1608–1611.
25 (a) L. Gong, X. Y. E. Chng, Y. Du, S. Xi and B. S. Yeo, ACS 30 D. Ressnig, Ph.D. Thesis, Universität Potsdam, Max Planck
Catal., 2018, 8, 807–814; (b) H.-Y. Wang, S.-F. Hung, Society, 2014.
H.-Y. Chen, T.-S. Chan, H. M. Chen and B. Liu, J. Am. 31 M. Krott, X. Liu, B. P. T. Fokwa, M. Speldrich, H. Lueken
Chem. Soc., 2016, 138, 36–39; (c) D. Friebel, M. Bajdich, and R. Dronskowski, Inorg. Chem., 2007, 46, 2204–2207.
B. S. Yeo, M. W. Louie, D. J. Miller, H. Sanchez Casalongue, 32 J. Fang, J. Qiao, D. P. Wilkinson and J. Zhan,
F. Mbuga, T.-C. Weng, D. Nordlund, D. Sokaras, R. Alonso-
Mori, A. T. Bell and A. Nilsson, Phys. Chem. Chem. Phys.,
2013, 15, 17460–17467.
Electrochemical Polymer Electrolyte Membranes, CRC Press,
[S.l.], 2017.
33 A. L. Tchougréeff, R. P. Stoffel, A. Houben, P. Jacobs,
R. Dronskowski, M. Pregelj, A. Zorko, D. Arčon and
O. Zaharko, J. Phys.: Condens. Matter, 2017, 29, 235701.
26 B. S. Yeo and A. T. Bell, J. Am. Chem. Soc., 2011, 133,
5587–5593.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2018