Evolution of Carbonate‐Intercalated γ‐NiOOH from a Molecularly Derived Nickel Sulfide (Pre)Catalyst for Efficient Water and Selective Organic Oxidation [original]

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Evolution of Carbonate-Intercalated γ-NiOOH from a

Molecularly Derived Nickel Sulfide (Pre)Catalyst for

Efficient Water and Selective Organic Oxidation

Suptish Ghosh, Basundhara Dasgupta, Shweta Kalra, Marten L. P. Ashton,

Ruotao Yang, Christopher J. Kueppers, Sena Gok, Eduardo Garcia Alonso,

Johannes Schmidt, Konstantin Laun, Ingo Zebger, Carsten Walter, Matthias Driess,

and Prashanth W. Menezes*

DOI: 10.1002/smll.202206679

the two-electron transfer hydrogen evo-

lution reaction (HER) at the cathode is

a fairly simple process, it is the sluggish

kinetics of the four-electron transfer OER

at the anode that acts as the bottleneck for

electrochemical water-splitting.[2] In the

recent past, numerous nickel-based elec-

trocatalysts, such as Ni oxides/hydroxides,

chalcogenides, pnictides, borides, and

carbides, with promising OER kinetics

and stability under different electrolyte

environments have been developed.[3]

These materials are cheap, abundant, and

non-toxic alternatives to benchmark noble

metal-based RuOx or IrOx catalysts and

have been regarded as suitable for large-

scale practical application.[4,5]

Amongst these materials, Ni chalcoge-

nides have attracted enormous attention

as highly efficient OER electrocatalysts due

to their excellent activity and stability, high

specific surface areas, structural diversity,

high conductivity, and facile redox tun-

ability of the catalytically active Ni centers to access multiple

oxidation states.[6] Studies have found that most of the Ni chal-

cogenide materials act as (pre)catalysts and transform (either

surface, partial, or bulk transformation) under OER conditions,

via the leaching of the chalcogen as oxyanions, resulting in an

The development of a competent (pre)catalyst for the oxygen evolution

reaction (OER) to produce green hydrogen is critical for a carbon-neutral

economy. In this aspect, the low-temperature, single-source precursor (SSP)

method allows the formation of highly efficient OER electrocatalysts, with

better control over their structural and electronic properties. Herein, a transi-

tion metal (TM) based chalcogenide material, nickel sulfide (NiS), is prepared

from a novel molecular complex [NiII(PyHS)4][OTf ]2 (1) and utilized as a (pre)

catalyst for OER. The NiS (pre)catalyst requires an overpotential of only

255mV to reach the benchmark current density of 10mAcm−2 and shows

63h of chronopotentiometry (CP) stability along with over 95% Faradaic

efficiency in 1m KOH. Several ex situ measurements and quasi in situ Raman

spectroscopy uncover that NiS irreversibly transformed to a carbonate-inter-

calated γ−NiOOH phase under the alkaline OER conditions, which serves as

the actual active structure for the OER. Additionally, this in situ formed active

phase successfully catalyzes the selective oxidation of alcohol, aldehyde,

and amine-based organic substrates to value-added chemicals, with high

efficiencies.

ReseaRch aRticle

S. Ghosh, B. Dasgupta, S. Kalra, M. L. P. Ashton, R. Yang, C. J. Kueppers,

S. Gok, E. G. Alonso, C. Walter, M. Driess, P. W. Menezes

Department of Chemistry

Metalorganics and Inorganic Materials

Technische Universität Berlin

Straße des 17. Juni 115, Sekr. C2 10623, Berlin, Germany

E-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article

can be found under https://doi.org/10.1002/smll.202206679.

J. Schmidt

Department of Chemistry

Functional Materials

Technische Universität Berlin

Hardenbergstraße 40 10623, Berlin, Germany

K. Laun, I. Zebger

Department of Chemistry

Physical Chemistry/Biophysical Chemistry

Technische Universität Berlin

Straße des 17. Juni 135, Sekr. PC14 10623, Berlin, Germany

P. W. Menezes

Materials Chemistry Group for Thin Film Catalysis – CatLab

Helmholtz-Zentrum Berlin für Materialien und Energie

Albert-Einstein-Str. 15 12489, Berlin, Germany

1. Introduction

Hydrogen production through water-splitting, driven by renew-

able energy-derived electricity, is a promising green and sustain-

able approach to meet the global energy demands.[1] Although

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open access article under the terms of the Creative Commons Attribu-

tion License, which permits use, distribution and reproduction in any

medium, provided the original work is properly cited.

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(oxy)hydroxide active phase, with higher electrochemical surface

area.[7] Such a transformation occurs as the oxidation potentials

of chalcogenides and Ni are much lower than that of water, and

has often been proven beneficial for OER in comparison to as-

synthesized, sacrificial anion-free NiOxHy catalysts.[8–10]

Nickel sulfides have been extensively developed as promising

electrocatalysts for OER due to their unique and adjustable

composition, structural and electronic features, and rich active

sites.[11] Moreover, both nickel and sulfur are cheap, abundant

and do not possess any significant hazard, especially sulfur,

which is an important (micro)nutrient for animals and plants.[9]

The activities of some nickel sulfides have been achieved close

to the benchmark noble-metal electrocatalysts, by incorporating

various surface modifications such as surface anion decora-

tion; phase, structure, and composition regulations, doping

with other elements, etc.[11] Therein, recently, nickel sulfide

nanowires stuffed into carbon nitride reported unprecedentedly

low overpotentials for OER.[6] This encourages the development

of novel nickel sulfide electrocatalysts, with unique chemical

and physical properties and high activities, and a more in-

depth understanding of the role of sulfur, factors affecting its

leaching, the variation in the active structures, etc.[3]

The traditional pathways to synthesize such Ni sulfides gen-

erally involve high-temperature solid-state methods, which are

not only energy-intensive but also form bulk material with

mixed phases, distinct morphologies, and large particles with

a varying size distribution, usually leading to poor activities.[12]

Therefore, alternative low-energy synthetic routes, which can

provide access to nanostructured materials with high sur-

face area, abundant catalytically active sites as well as display

practical performance are desirable. In this regard, the low-

temperature SSP approach for the synthesis of electrocatalysts

has shown striking advantages over the conventional high-

temperature approaches including improved control over the

composition of the material, uniformity of morphology and

particle size, and a homogenous distribution of the elements

throughout the bulk of the material.[12] In this respect, several

SSP-derived materials have recently been utilized for OER and

have shown promising results.[13–20]

Recently, considerable efforts have also been dedicated

toward replacing the OER with kinetically less demanding

organic oxidation reactions (hybrid-water electrolysis), which is

a promising approach toward a) decreasing the overpotential of

the overall water-splitting system, b) the simultaneous genera-

tion of value-added products at the anode, and hydrogen fuel

at the cathode, thus making the process more economical, and

c) increasing the safety by avoiding the formation of explosive

H2/O2 gas mixture.[21–23] In this regard, several OER electrocata-

lysts have also been directly used for organic transformations to

yield value-added products, since similar active sites are known

to catalyze both OER and organic oxidations.[24] The electrocata-

lytic route avoids the use of toxic and expensive reagents/cata-

lysts and harsh conditions and therefore is a promising alterna-

tive sustainable method to chemical synthesis.[25–27]

In view of the above, the current work addresses and answers

the following scientific questions: i) can we develop a facile

synthetic approach to derive NiS from a molecular precursor?,

ii) what is the reconstructed active structure of the NiS (pre)

catalyst for OER?, iii) how does the active structure influence

the electrocatalytic OER activity? and iv) is the active phase also

suitable for the selective oxidation of organic substrates?

Herein, for the first time, we successfully designed a

tetrakis(pyridiniumsulfido) Ni-complex, [NiII(PyHS)4][OTf]2 (1),

using a simple one-step synthetic protocol, which turned out

to be a suitable SSP for the synthesis of nanostructured NiS,

using the hot-injection method (Scheme1). The synthesized

NiS material acts as a highly efficient OER electro(pre)catalyst

in alkaline conditions. NiS undergoes a complete reconstruc-

tion under the alkaline OER, via the leaching of sulfur into the

electrolyte, to form a carbonate-intercalated γ-NiOOH phase,

with an interlayer spacing of ≈7.7 Å, which serves as the actual

active phase for OER. The OER activity of the SSP-derived NiS

(pre)catalyst can be attributed to the high electrochemical sur-

face area (ECSA), the number of Ni redox-active sites, and turn-

over frequency (TOF). We have also utilized our SSP-derived

NiS (pre)catalyst to perform the selective oxidation of 5-hydrox-

ymethylfurfural (HMF), benzyl alcohol (BAl), and benzylamine

(BAm) to value added-chemicals with high efficiencies. There-

fore, the current work demonstrates a novel molecular pre-

cursor for the synthesis of NiS nanoparticles, which serves as

an excellent OER as well as a versatile organic oxidation (pre)

catalyst.

2. Results and Discussion

2.1. Synthesis and Characterization of [NiII(PyHS)4][OTf ]2 (1)

Precursor

The molecular precursor complex 1 was synthesized (see details

in the Supporting Information) and isolated as a crystalline

brown powder with 85% yield by reacting nickel(II) triflate

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Scheme 1. Synthesis of the nickel (II) complex 1, [NiII(PyHS)4][OTf]2, using 2-mercaptopyridine as supporting ligand. The 2-mercaptopyridine ligand

exists in equilibrium with its tautomers such as 2-pyridine-thiol (PyHS), 2-pyridine-thione (PyHS),[28] and a zwitterionic structure (PyH+S−).[29] The

square planar {NiIIS4} core in 1 was used as a SSP to prepare the crystalline NiS nanostructure material (Ni: green; S: orange).

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and 2-mercaptopyridine (with a molar ratio of 1:4) in dichlo-

romethane (Scheme 1). The isolated complex 1 was character-

ized by the elemental analysis, 1H and 19F NMR spectroscopy,

electrospray ionization mass spectrometry (ESI MS), Fourier

transformed infrared spectroscopy (FT-IR), and single-crystal

X-ray diffraction studies (Figures S1–S5, Tables S1 and S2, Sup-

porting Information). The 1H NMR spectrum of 1 in CD2Cl2

showed resonances of the chemically indistinguishable aro-

matic protons at the pyridine ring of the 2-mercaptothiol

ligand within the range of 6.79 to 13.06ppm, which explicitly

confirms the diamagnetic nature of the complex (Figure S1,

Supporting Information). The ESI MS spectrum of 1 (in tet-

rahydrofuran, positive ion mode) displayed major molecular

ion peaks at m/z values of 389.97, 428.92, and 539.93 with an

expected isotopic distribution calculated for the molecular

fragments, [{Ni(PyHS)3}-H]+, [{Ni(PyHS)2(PyS)}+K]+ and

[{Ni(PyHS)2(PyS)2}+K]+ respectively (Figure S3, Supporting

Information). The FT-IR of the isolated complex 1 features two

strong vibrations at 1131 and 1363cm−1 can be assigned to the

vibrations of ν(SO) and ν(CF), respectively, of the triflate

counter anion (Figure S4, Supporting Information).

Complex 1 adopts a monoclinic space group P21/c (Table S1,

Supporting Information). X-ray structure analysis confirmed

the presence of two triflate counter ions along with a tetraco-

ordinate nickel (II) as a dicationic entity. The unit consists of a

nearly square-planar {NiIIS4} core coordinated by four terminal

monodentate PyHS ligands (Figure 1 and Figure S5, Sup-

porting Information). The {NiIIS4} core is symmetrical, with

NiS bond distances and SNiS bond angles in the range of

2.2037(5)–2.2180(4) Å and 97.307(17)–82.694(17)°, respectively,

which are in good agreement with the previously reported

nickel sulfide complexes (Table S2, Supporting Information).

2.2. Synthesis and Characterization of NiS Nanoparticles

Thermal decomposition of complex 1 at 275 °C in oleylamine

for 2 h led to the formation of NiS nanoparticles (termed as

NiS-M hereafter; see supporting information for details). The

powder X-ray diffraction (pXRD) of the as-synthesized mate-

rial revealed the formation of a highly crystalline and pure NiS

phase (JCPDS No. 01-086-2281) (Figure S6, Supporting Infor-

mation). The NiS-M adopts a rhombohedral crystal system

with R3m space group (no. 160) (Figure 2a, Supporting Infor-

mation). The Raman spectrum of NiS-M showed four dis-

tinct peaks at 249, 303, 351, and 373 cm−1 which corresponds

to the stretching modes with A1, A1, E, and A1 symmetries,

respectively (Figure 2b).[30] The morphology was then exam-

ined by scanning electron microscopy (SEM), where the as-

prepared NiS-M exhibits hexagonal-type particles. (Figure 2c

and Figure S7, Supporting Information). Elemental mapping

suggests a homogeneous distribution of Ni and S throughout

the material (Figure 2d and Figure S8, Supporting Informa-

tion). Quantification by energy-dispersive X-ray spectroscopy

(EDX) revealed a ≈1:1 ratio of Ni and S, with <1% of oxygen

content (Figure S9, Supporting Information). Furthermore,

inductively coupled plasma atomic emission spectroscopy

(ICP-AES) of NiS-M also confirmed the ratio of Ni and S as

1:1 (see Table S4, Supporting Information). The transmission

electron microscopy (TEM) image showed the hexagonal-type

morphology of NiS-M, which further supports the finding from

SEM (Figure2e and Figure S10, Supporting Information).

To understand the chemical state of NiS-M, X-ray photoelec-

tron spectroscopy (XPS) was performed with a narrow scan of

Ni 2p, S 2p, and O 1s. The deconvoluted Ni 2p spectrum reveals

the presence of Ni2+ peaks at 853.3 and 870.8eV that could be

assigned to 2p3/2 and 2p1/2, respectively.[31] However, the addi-

tional peaks at 855.9 eV (2p3/2) and 873.4 eV (2p1/2) binding

energies could be ascribed to Ni(OH)2 species due to surface

passivation.[32] The appearance of the satellite signals also con-

firmed the presence of Ni2+ species in NiS-M (Figure2f). The

deconvoluted S 2p spectrum showed the presence of sulfide ion

(S2−) containing 2p3/2 (161.8eV) and 2p1/2 (163.2eV) along with

disulfide (S22−) species 2p3/2 (164.8 eV) and 2p1/2 (166.8 eV).

The disulfide species forms due to the surface passivation to a

Ni(OH)2 species creating Ni deficiencies in the underlying NiS,

thus bringing the stoichiometry close to NiS2, as described in

previous studies.[33] Additionally, due to surface passivation, a

higher oxidation state (SO42−) was also observed, wherein the

peaks can be assigned to 2p3/2 (167.5eV) and 2p1/2 (168.8eV)

of the SO bond (Figure2g).[33,34] The O 1s XPS spectra also

confirmed the presence of the aforementioned surface oxidized

species (Figure S11, Supporting Information).[35]

In order to substantiate the advantages of the SSP method

over traditional methods for synthesizing a superior water-

oxidation catalyst, we also synthesized NiS via a hydrothermal

method (NiS-H). In addition, as layered oxyhydroxides are

known to be the final active structure for OER among Ni-

based sulfides, we deliberately prepared a NiOOH via a wet

chemical approach and used it as a reference. The detailed

state-of-the-art characterization for both materials is provided in

Figures S12–S16, Supporting Information.

2.3. Electrochemical OER Activity

Prior to the OER measurements, all the catalysts were depos-

ited on a 3D porous nickel foam (NF) and fluorine-doped tin

oxide (FTO) substrates using electrophoretic deposition (EPD)

(see details in Supporting Information), which were used as the

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Figure 1. Molecular structure of the dication of 1. The molecular struc-

ture has been depicted with thermal ellipsoids at 50% probability. Triflate

anions are omitted for clarity. Color code; carbon: gray, nitrogen: blue,

sulfide: orange, nickel: green.

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working electrode for all the electrochemical measurements.

The mass loading of all the catalysts was controlled by varying

the EPD time to obtain the same loading (0.8 ± 0.1mgcm−2

on NF and 0.4 ± 0.1mgcm−2 on FTO). Thereafter, the depos-

ited films were characterized by SEM and ICP-AES, which con-

firmed that the morphology and composition of the materials

remain unaltered during the deposition process (Table S4, Fig-

ures S17 and S18, Supporting Information). Before evaluating

the electrochemical activities, the catalysts were first activated

by performing at least 10 cycles of cyclic voltammetry (CV)

in the anodic region (with a scan speed of 5mVs−1, between

1.1–1.75 VRHE) until stable and reproducible current responses

were obtained. All the catalysts showed a unique redox feature

in their respective CVs, between 1.2–1.4 VRHE, wherein in the

anodic sweep, Ni is oxidized to a higher oxidation state (i.e.,

Ni2+→ Ni3+), while in the cathodic sweep it reverts back to its

parent state (i.e., Ni3+→ Ni2+) (Figure S19, Supporting Informa-

tion). This redox feature has been observed previously for other

Ni-based materials as well.[14,26,36]

As shown in Figure 3a, the catalytic activities were esti-

mated from the iR-compensated CV curves, recorded at a

scan rate of 2 mV s−1 to achieve a close-to steady-state cur-

rent. The required overpotential (η) for NiS-M/NF to reach

the benchmark current density of 10 mA cm−2 was only

255 ± 2mV, which is significantly lower than NiS-H/NF (300 ±

3mV), NiOOH/NF (350 ± 5mV) and bare NF (375 ± 5mV),

respectively. Furthermore, we compared the OER activity with

noble metal-based state-of-the-art catalysts and performed the

linear sweep voltammetry (LSV) under identical OER condi-

tions. Impressively, NiS-M/NF delivers the best result when

directly compared to RuO2 and IrO2 (Figure S20, Supporting

Information). The reaction kinetics were examined by the

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Figure 2. a) Crystal structure of as-prepared NiS-M depicting Ni (green spheres) and S (orange spheres) in a unit cell (gray lines). b) Raman spectra

of NiS-M (200–500 cm−1) showing characteristic peaks at 249, 303, and 373 for A1 and 351 cm−1 for E symmetries. c) SEM image of NiS-M, showing

hexagonal-type particles and the corresponding elemental mapping d) shows a homogeneous distribution of Ni (blue) and S (green) in NiS, e) TEM

image of NiS-M showing a hexagonal-type particle, f) Ni 2p and g) S 2p XPS spectra of NiS-M showing the presence of Ni2+ and S2− oxidation states

of NiS phase (satellite peaks are represented by *), respectively.

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steady-state Tafel slope measurement, which reveals that

NiS-M/NF has the lowest Tafel slope value of 76.2mVdec−1,

followed by NiS-H/NF (92.7 mV dec−1) and NiOOH/NF

(124.5 mV dec−1), respectively, which indicated that NiS-M

has the lowest kinetic barrier (Figure3b). To further corrob-

orate the activity trend, the charge transfer resistance of the

catalysts (Rct) was measured using electrochemical impedance

spectroscopy (EIS), where the obtained semi-circle curves

were fitted with an equivalent Randle circuit (Figure3c and

Figure S21, Supporting Information). As expected, NiS-M/

NF showed the lowest Rct value of 2.21 Ω, followed by NiS-H/

NF (3.88 Ω) and NiOOH/NF (7.02 Ω), respectively (see Sup-

porting Information Table S3, Supporting Information).

These findings indicated that the SSP-derived catalyst, NiS-M,

is governed by faster electron transfer between the catalyst

and the electrolyte as compared to other materials, which

manifests in its highest OER performance. Furthermore, to

estimate the ECSA of the catalysts, the double layer capaci-

tance (Cdl) was measured (as Cdl is directly proportional to

the ECSA) by recording CVs in the non-Faradaic region, with

increasing scan speeds (Figure S22, Supporting Informa-

tion).[37] The estimated Cdl values after OER were 10.81, 6.28,

and 1.07 mFcm−2 for NiS-M/NF, NiS-H/NF, and NiOOH/NF,

respectively, which are in agreement with the activity trend of

the catalysts (Figure3d). Additionally, the Cdl value of NiS-M/

NF increased by ninefold after CV activation (Cdl before OER

is 1.13 mFcm−2), indicating that the material is transformed

during OER (Figure S23, Supporting Information).

Furthermore, the number of Ni redox-active sites was

obtained by the integration of the redox peaks in the cathodic

sweep; wherein a higher number is observed for NiS-M/NF as

compared to NiS-H/NF, reinforcing the superior OER activity

of NiS-M over NiS-H (Figure S24, Supporting Information).

The stability of the best-performing catalyst, NiS-M, was tested

by performing CP at a constant current density of 10mAcm−2.

NiS-M/NF displayed over ≈63h of stable activity, without any

deterioration (Figure3e). The outstanding stability of this SSP-

derived catalyst encouraged us to measure its reliability under

a high current density of 500 mA cm−2, to fortify the scope

for industrial implementation. NiS-M/NF showed stable per-

formance for ≈10h at this high current density under testing

conditions (Figure S25, Supporting Information). In order to

comprehend the intrinsic activity of the catalysts, the steady-

state OER activity of the catalysts was normalized by the Cdl

values and the number of Ni redox-active sites. Both the Cdl

and number of Ni redox electrons normalized CVs of NiS-M

displayed a better activity over the NiS-H, on NF, which is in

agreement with the Tafel slope trend (Figure3b and Figure S26,

Supporting Information), indicating that the activity per active

site of NiS-M is superior to that of NiS-H. Furthermore, the

TOF is considered as one of the most dependable parameters

to evaluate the intrinsic activity of the catalyst (see details in

the Supporting information).[39,40] The obtained TOF value

of NiS-M was 0.1 s−1, which was two magnitudes higher than

that for NiS-H (0.05 s−1), and further supports the Cdl and Ni

redox-active sites normalized results. Finally, an OER Faradaic

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Figure 3. a) OER activity comparison of NiS-M, NiS-H, NiOOH electrodes on NF and bare NF in 1m KOH (pH = 13.89)[38] at 25 °C from CV curves at a

scan rate of 2mVs−1. b) Tafel slopes for NiS-M, NiS-H, and NiOOH on NF from steady-state measurements. c) EIS spectra of the materials measured

at 1.52 VRHE. The spectra were fitted using a Randle circuit (see Table S3, Supporting Information, for fitting values). d) Cdl value determination from

CV scans in a non-Faradaic potential range. e) CP measurement of NiS-M/NF at 10mAcm−2 for ≈63h. f) OER activity comparison of NiS-M, NiS-H,

NiOOH on FTO and bare FTO electrodes in 1m KOH at 25 °C from CV curves at a scan rate of 2mVs−1.

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efficiency of 95 ± 1% was obtained for NiS-M, which reiterates

its scope for commercial application (Figure S27, Supporting

Information).

Nonetheless, the use of NF as a substrate often contributes

to the catalytic activity,[27] and to rule out this discrepancy, we

further carried out the electrochemical studies on the FTO sub-

strate under identical conditions. As shown in Figure 3f, the

polarization curves of the catalysts follow the same trend of

activity as on NF. The required overpotentials (η10)are 350 ±

2, 385 ± 3, and 420 ± 4mV for NiS-M/FTO, NiS-H/FTO, and

NiOOH/FTO, respectively (Table S5, Supporting Information).

The post-OER Cdl, EIS, number of Ni redox-active sites, and the

activity from normalized CV curves on FTO also showed the

same trend as on NF (Figures S28–S31, Supporting Informa-

tion). Furthermore, NiS-M/FTO has remarkably stable activity

at 10mAcm−2 for ≈24h, thereby indicating the robustness of

the catalyst (Figure S32, Supporting Information). For a better

comparison, NiS-H/FTO was also subjected to the stability test

under 10 mA cm−2 current density, and a clear decay in the

activity is observed (Figure S33, Supporting Information).

In summary, the superior activity of the SSP-derived NiS-M

(pre)catalyst can be ascribed to its higher Cdl value and the

higher number of redox-active sites participating in OER.

Furthermore, the lower Tafel slope of NiS-M suggests faster

reaction kinetics, that is, a small increase of potential results

in a higher increase in exchange current density for NiS-M

as compared to the other catalysts. Concomitantly, the higher

intrinsic activity of NiS-M is further authenticated by its higher

TOF value and the trend from the normalized CVs. Therefore,

NiS-M proves as a superior (pre)catalyst for alkaline water oxi-

dation reactions and its activities are comparable with previ-

ously reported Ni-based electrocatalysts (Table S6, Supporting

Information).

2.4. Post OER Ex Situ Characterization

In order to elucidate the structural changes of NiS-M under

electrochemical OER conditions, SEM, elemental mapping,

EDX, TEM, and HR-TEM were measured after 24h OER CP

measurement at 10 mA cm−2. A microstructural reformation

was observed in the post-OER SEM images, wherein the hex-

agonal architecture was transformed to a sheet-like morphology

(Figure S34, Supporting Information). The EDX spectrum and

ICP-AES indicate complete S leaching from the catalyst (Fig-

ures S35 and S36, Table S4, Supporting Information), which

dissolved as highly water soluble SO42− anions (according to

the Pourbaix diagram).[41,42] Such anion leaching has also been

observed previously for other Ni-based (pre)catalysts.[7] A trace

amount of Fe (≈ 1%) is observed from the EDX spectrum due to

its presence in the KOH electrolyte as impurities (Figures S35

and S36, Supporting Information), which often contributes

to enhancing the activity of Ni-based materials.[43] Elemental

mapping shows a homogeneous distribution of Ni and O, indi-

cating the formation of oxidic Ni active phase and the presence

of adsorbed K on the surface, which could be due to residual

KOH electrolyte (Figure S37, Supporting Information).[44] Addi-

tionally, the EDX spectrum after CP at 500mAcm−2 for 10h

also reveals complete leaching of S, which is consistent with

the findings after 24h OER CP at 10mAcm−2 (Figures S38 and

S39, Supporting Information). Furthermore, a close examina-

tion of TEM images corroborates the microstructural transfor-

mation, revealing a sheet-like morphology after OER, like the

findings from SEM images (Figure 4a, Figures S34 and S40,

Supporting Information). The HR-TEM image shows well-

resolved lattice fringes, with an interplanar distance of 7.7 Å for

the plane (003) of γ−NiOOH (JCPDS No. 00-06-0075), which is

further confirmed by the fast Fourier transform (FFT) images

(Figure4b). To unveil the chemical state after OER of NiS-M,

XPS spectra were recorded after 24h OER CP. The narrow scan

of the Ni 2p spectrum reveals a peak at 855.6eV (Ni 2p3/2) and

873.3eV (Ni 2p1/2) which could be deconvoluted to Ni2+ and Ni3+

species, suggesting the presence of both these species in the

catalyst after electrochemical treatment.[31] The binding energy

value of Ni3+ species was similar to the earlier reported values

for the γ−NiOOH phase.[45,46] Furthermore, the co-existence of

satellite peaks at 861.3eV and 879.6eV also authenticates this

finding (Figure4c). [47] As expected, the S 2p spectrum does not

show any signal, which signifies a complete S leaching from

the material and substantiates our findings from post-OER

EDX analysis (Figures S35 and S41, Supporting Information).

However, a trace amount of S (0.02%) was also detected in ICP-

AES analysis after 24h of the CP experiment which could be

due to the re-adsorption or trapped oxyanion that possibly can

improve the kinetics by stabilizing OER reaction intermedi-

ates (Table S4, Supporting Information).[48] Furthermore, the

O 1s narrow scan shows three main characteristic peaks corre-

sponding to lattice oxygen (O1), hydroxyl/(oxy)hydroxyl groups

(O2), and carbonates (O3), respectively (Figure4d).[49–53] The C

1s spectrum reveals peaks at 284.8, 286.8, and 288.7eV corre-

sponding to the CC, CO, and OCO bonds, respectively

(Figure4e).[54] The peak at 288.7eV confirmed the presence of

carbonate anion (CO32−) within the catalyst,.[55] The appearance

of the new peaks at 292.7 and 295.5eV is due to adsorbed K+,

from the KOH electrolyte during the electrochemical measure-

ment.[44] The FT-IR spectrum after 24h OER CP measurement

also confirmed the presence of CO32− in the in situ formed

γ−NiOOH (Figure S42, Supporting Information).

2.5. In Situ Raman Spectroscopy

To gain further in-depth information on the structural transfor-

mation of NiS-M during the OER, in situ (resonance) Raman

spectroscopy was employed as it is considered as a dependable

probe to reveal the true active structure of a catalyst.[14,56–58]

For this, NiS-M was freeze-quenched in liquid N2 (at −196 °C)

after 24h OER CP at 10mAcm−2 (see Supporting Information

for details). As shown in Figure4f, the quasi in situ spectrum

displays two major Raman peaks at 481 and 560 cm−1.[57–59]

Comparing with previously reported literature, the first peak

(481 cm−1) corresponds to the depolarized bending mode

δ(NiIIIO), and the second peak (560 cm−1) corresponds to the

polarized stretching mode ν(NiIIIO) of γ−NiOOH.[57,60] Along

with this, the other peaks at 1065 cm−1 arise from the CO32−

(dissolved in the electrolyte from ambient air).[61,62] Similar

peaks were also observed in the ex situ spectrum.[63] Further-

more, the broad peak between 800–1000 cm−1 and the peak at

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1079 cm−1 in the in situ spectrum has been previously attrib-

uted to ν(OO) of the active oxygen species (NiOO−) in the

NiOOH.[57,58,64] In the ex situ spectrum, two extra peaks at 417

and 503 cm−1 were also detected, corresponding to the non-sto-

ichiometric [NiO] stretching vibration of NiO.[65,66] However,

these two signals were missing in the in situ spectrum, indi-

cating oxidation of the Ni surface under air in ex situ condi-

tions. For Raman spectra of the as-prepared and FTO-deposited

NiS-M samples, no vibrational peaks corresponding to oxidized

species were detected. Nonetheless, the Raman spectroscopy

analysis supports the formation of a γ−NiOOH active phase

with intercalation of CO32- in the lattice spacing and is also con-

sistent with the recent report of Hunter etal. who reported that

CO32− intercalation in NiFe-layered double hydroxides results in

a basal spacing close to 7.7 Å.[67]

2.6. Active Structure for OER

It is well-established that under oxidation potential in an alka-

line environment, Ni-based chalcogenide materials are often

oxidized to a nickel (oxy)hydroxide active phase via the leaching

of the chalcogen atoms as water-soluble oxyanions, resulting

in a porous and high-surface area skeletal Ni catalyst.[6,9] This

phase transformation under the OER operational conditions

has previously been well-described by Bode’s diagram.[68] Such

beneficial leaching is also extensively applied in industries for

the production of Raney nickel via the leaching of Al from Ni-Al

alloy.[69] Therefore, consistent with these previous findings,

we also observed a complete and fast leaching of S from our

NiS-M (pre)catalyst under alkaline OER conditions, resulting

in the formation of a CO32−-intercalated γ-NiOOH active phase,

as confirmed by Raman spectroscopy and XPS (Figure 5).

CO32− anions dissolved in electrolyte from ambient air have

been reported to rapidly replace all other intercalated anions

in an LDH structure due to its highest ion-exchange equilib-

rium constant.[43] Such unprecedented intercalation of CO32−,

resulted in a larger interlayer spacing (≈ 7.7 Å) between the

[NiO6] layers of γ−NiOOH, as confirmed by the HR-TEM than

the usual expected inter-layer spacing between [NiO6] layers of

6.9 Å.67This has been already well described before in the liter-

ature and is known to accelerate the reaction rate due to higher

penetration of OH− anions into the bulk of the catalyst, thus

facilitating the OO bond formation .[14]

2.7. Value-Added Selective Oxidation of Organic Substrates

We further applied our NiS-M (pre)catalyst for the selective oxi-

dation of hydroxymethylfurfural (HMF), benzyl alcohol (BAl)

and benzylamine (BAm) to value-added products 2,5-Furandi-

carboxylic acid (FDCA), benzoic acid, and benzonitrile, respec-

tively, by coupling it with the hydrogen evolution reaction (HER)

at the anode in a three-electrode undivided cell, (Figure 6a). It is

worth noting here that FDCA (produced from biomass-derived

HMF), benzoic acid (with an annual production of 648 kt)[70] and

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Figure 4. a) TEM image and corresponding b) SAED and FFT (inset) of NiS-M after OER CP. c) Ni 2p, d) O 1s, and e) C 1s XPS spectra of NiS-M after

OER CP (satellite peaks are represented by *). f) Raman spectra of NiS-M powder, as-deposited NiS-M/FTO, and after OER CP (both in situ and ex

situ). The results from Raman, TEM, and XPS confirm the formation of CO32−-intercalated γ-NiOOH active phase after OER.

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benzonitrile (and other amine-based compounds), are widely

used as starting materials in polymer, chemical, and pharma-

ceutical industries, respectively.[71–74] Therefore, to achieve these

value-added selective oxidations, we first completely recon-

structed the NiS-M/NF electrode into the stable γ-NiOOH active

phase by performing continuous CV cycles in the anodic region

(1.1–1.75 VRHE), since it is well-known that similar active sites

catalyze both water and organic oxidation reaction.[24] After that,

LSV was recorded for the NiS-M/NF electrode in 1m KOH in

the absence and presence of 0.1m of HMF, Bal, and BAm, indi-

vidually, without stirring (Figure6b). Interestingly, we observed

that the onset of oxidation of all three substrates occurs prior

to OER, with a large redox peak (Ni2+→ Ni3+) between 1.35

and 1.5 VRHE, followed by a much steeper increase in current

density as compared to the OER.[26] Therefore, the organic

oxidations occur exclusively between 1.38–1.5 VRHE, with neg-

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Figure 6. a) Bulk oxidation of reductive chemicals (HMF, BAl, and BAm) to value-added products (FDCA, benzoic acid, and benzonitrile) at the anode

(NiS-M/NF), coupled with HER at the cathode (Pt wire) in 1m KOH via an undivided cell. b) LSV curves of NiS-M in 1m KOH (blue) and along with

0.1m of HMF (red)/BAl (yellow)/ BAm(green) substrates (without stirring). Bulk CA electrolysis at 1.49 VRHE, with stirring of c) HMF oxidation, d) BAl

oxidation, and e) BAm oxidation, which was terminated after exactly the charge required for full conversion was passed. 1H NMR spectrum (200MHz,

D2O) of reaction mixture before and after CA of f) HMF oxidation, g) BAl oxidation, and h) BAm oxidation. The blue dotted lines represent the reactant

peaks and the pink dotted lines represent the product peaks.

Figure 5. Reconstruction of NiS-M, under oxidizing anodic potential, via leaching of S and intercalation of CO32− from ambient air, to form a CO32−

intercalated γ-NiOOH active structure, which catalyzes the OER.

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ligible OER current response, which indicates NiS-M/NF is

capable of catalyzing the reaction of substrates at the anode in

this potential range. We also performed the LSVs of organic

oxidation for bare NF and observed a very low activity for all

the three substrates (Figure S43, Supporting Information). In

order to characterize and quantify the products, we performed

bulk chronoamperometry (CA) electrolysis of the substrates at

1.49 VRHE, just before OER onset, with 0.1m of the substrates,

in 1m KOH, with stirring (Figure6c–e). Under this steady state

condition, the organic oxidation reactions reach high current

densities, with ≈ 550mAcm−2 for HMF oxidation (Figure6c),

≈300mAcm−2 for BAl oxidation (Figure6d) and ≈600mAcm−2

for BAm oxidation (Figure6e), which decreased over time due

to substrate consumption. 1H NMR spectrum of the reaction

mixtures was recorded after passing exactly the charge that is

required for complete conversion (which is 868.4 C for HMF,

160.8 C for BAl and BAm) and the obtained NMR signals exclu-

sively belonged to the products, that is, FDCA (Figure6f and

Figure S44, Supporting Information), benzoic acid (Figure6g

and Figure S46, Supporting Information) and benzylamine

(Figure 6h and Figure S48, Supporting Information). How-

ever, to quantify the product yield, maleic acid was used as the

internal standard. The determined yield of FDCA was ≈79%

(Figure S45, Supporting Information) and benzoic acid was

≈ 87% (Figure S47, Supporting Information). As benzonitrile

remains as a partially soluble oily product in the electrolyte,[75]

we isolated the product and obtained a yield of ≈80%.

Herein, in the 1H NMR spectra recorded after CA, the signal

at 4.7ppm is of the residual water from 1m KOH, while the

signal at 5.9ppm represents the internal standard (maleic acid)

for HMF and BAl oxidation (Figures6f,g). After CA measure-

ment of HMF oxidation, the signal corresponding to formate

(8.4ppm) due to the degradation of FDCA.[76] Consecutively, the

Faradaic efficiencies obtained for HMF oxidation is 79%, BAl

oxidation is 87% and BAm oxidation is 80%. We also performed

the CA of HMF oxidation for bare NF, which showed a very

low current density, and after 106min, the charge passed was

only 258.9 C and the NMR revealed only 12% of FDCA forma-

tion, along with a large amount of unreacted HMF (Figures S49

and S50, Supporting Information). Therefore, the high current

densities and Faradaic efficiencies for HMF, BAl and BAm

oxidation using NiS-M as (pre)catalyst suggests that the scope

of our material can be expanded further for the electrooxida-

tion of other valuable alcohol, aldehyde, and amine-containing

substrates.[75,77]

3. Conclusion

In summary, we successfully addressed all the research ques-

tions which were raised in the introduction. With respect to

question (i), we have successfully adopted a low-temperature

rationalized approach to synthesize an efficient NiS (pre)cat-

alyst from a novel molecular precursor for water oxidation.

The obtained catalyst displayed low overpotential (η10), charge

transfer resistance, and Tafel slope value as well as improved

intrinsic properties, which signifies the superiority of the SSP-

derived NiS for OER. To address the question (ii), scrupulous

ex situ and in situ characterization of the post OER electrode

were conducted that revealed the transformation of NiS to a

γ−NiOOH phase with CO32- intercalation between the [NiO6]

layers, which serves as the active structure for the OER. Such

CO32− intercalation in γ−NiOOH resulted in a large interlayer

spacing (≈7.7 Å) between the layers, which accelerates the pen-

etration of the OH− anions into the bulk of the catalyst, thus

enhancing its activity and answers the question (iii). Finally, to

answer the last question (iv), the in situ formed active phase

was also successfully employed to catalyze the selective oxi-

dation of alcohol, aldehyde, and amine-containing substrates

that displayed high yield and Faradaic efficiency. Therefore,

we believe this work establishes the pivotal role of a SSP to

access efficient electrocatalysts for OER, as well as organic

electrooxidation.

Supporting Information

Supporting Information is available from the Wiley Online Library or

from the author.

Acknowledgements

S.G. and B.D. contributed equally to this work. Funded by the Deutsche

Forschungsgemeinschaft (DFG, German Research Foundation) under

Germany’s Excellence Strategy – EXC 2008/1 – 390540038 – UniSysCat

and the German Federal Ministry of Education and Research (BMBF

project “PrometH2eus,” 03HY105C). P.W.M. greatly acknowledges

support from the German Federal Ministry of Education and Research

in the framework of the project Catlab (03EW0015A/B). S.K and

K.L are thankful to Einstein Foundation Berlin/EC2/BIG-NSE for a

Ph.D. fellowship, respectively. The authors are indebted to Christoph

Fahrenson (ZELMI, TU Berlin) for SEM, and Eva Maria Heppke, and Ina

Speckmann (TU Berlin) for pXRD measurements. The authors thank Dr.

Indranil Mondal and J. N. Hausmann for SEM and TEM measurements,

respectively. The authors thank Paula Nixdorf (TU Berlin) for single-

crystal XRD measurements and structure refinement and also thank

Dr. B. Chakraborty for introducing the 2-mercaptopyridine ligand for the

synthesis of SSPs.

Open access funding enabled and organized by Projekt DEAL.

Conflict of Interest

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available in the

supplementary material of this article.

Keywords

active γ-NiOOH phase, CO32−-intercalation, nickel sulfide, oxygen

evolution reaction, selective organic oxidation, single-source precursors

Received: October 29, 2022

Revised: December 8, 2022

Published online: January 18, 2023

www.advancedsciencenews.com www.small-journal.com

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