Inception of Co3O4as Microstructural Support to Promote Alkaline Oxygen Evolution Reaction for Co0.85Se/Co9Se8Network

Oxygen Evolution Reaction for Co Se/Co Se Network

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Inception of Co O as Microstructural Support to Promote Alkaline

.85

†

Sourav Ghosh,* Gouri Tudu, Ayan Mondal, Sagar Ganguli, Harish Reddy Inta,

and Venkataramanan Mahalingam*

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c02618

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sı Supporting Information

ABSTRACT: Developing electrocatalysts with abundant active sites is a substantial challenge to reduce the overpotential

requirement for the alkaline oxygen evolution reaction (OER). In this work, we have aimed to improve the catalytic activity of cobalt

selenides by growing them over the self-supported Co O microrods. Initially, Co O microrods were synthesized through annealing

of an as-prepared cobalt oxalate precursor. The subsequent selenization of Co O resulted in the formation of a grainy rodlike

Co O /Co Se/Co Se network. The structural and morphological analysis reveals the presence of Co O even after the

0.85

selenization treatment where the cobalt selenide nanograins are randomly covered over the Co O support. The resultant electrode

shows superior electrocatalytic activity toward OER in alkaline medium by delivering a benchmark current density of 10 mA/cm

geo

at an overpotential of 330 mV. As a comparison, we have developed Co₀_.₈₅Se/Co Se under similar conditions and evaluated its OER

activity. This material consumes an overpotential of 360 mV to deliver the benchmark current density, which signi�es the role of the

Co O support to improve the electrocatalytic activity of Co Se/Co Se . Despite having a low TOF value for Co O /Co₀_.₈₅Se/

0.85

−1

−

Co Se (0.0076 s ) compared to Co Se/Co Se (0.0102 s ), the improved catalytic activity of Co O /Co Se/Co Se is

0.85

attributed to the presence of a higher number of active sites rather than the improved per site activity. This is further supported from

the C (double layer capacitance) measurements where Co O /Co Se/Co Se and Co Se/Co Se tender C values of about

0.85

.19 and 1.08 mF/cm , respectively, after electrochemical precondition. As-prepared Co O /Co Se/Co Se also manifests rapid

3 4 0.85 9 8

kinetics (low Tafel slope � 91 mV/dec), long-term stability, low charge-transfer resistance, and 82% Faradaic e�ciency for alkaline

electrocatalysis (pH = 14). Furthermore, the proton reaction order (�RHE) is found to be 0.65, indicating a proton decoupled

electron transfer (PDET) mechanism for alkaline OER. Thus, the Co O support helps in the exposure of more catalytic sites of

Co₀_.₈₅Se/Co Se to deliver the improved catalytic activities in alkaline medium.

. INTRODUCTION

considered as state-of-the art catalysts in acidic and alkaline

mediums, respectively. High market price and scarceness

hinder their industrial implementation for commercial

The day-to-day requirement of clean energy has been outraised

with the green revolution. Consumption of fossil fuel dispels

toxic compounds that limits their real-world execution for a

practice.

Nonprecious cobalt-containing electrocatalysts

sustainable system. To date, hydrogen is in a commanding

position due to carbon-free emission and high-energy output.

In general, water electrolysis is one of the most convenient

routes for hydrogen production. However, the complex

pathway of the four-electron mechanism demands serious

overpotentials for the electrochemical oxygen evolution

Received: September 2, 2020

reaction (OER). In this regard, IrO and RuO are still

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 1. (a) PXRD patterns of as-prepared samples of (i) Co O (sample S2), (ii) Co O /Co Se/Co Se (sample S4), and (iii) Co₀_.₈₅Se/

0.85

Co Se (sample S5) along with standard pattern of Co O (JCPDS File # 43-1003), Co Se (JCPDS File # 01-089-4180), and Co₀_.₈₅Se (JCPDS

File # 52-1008). XPS patterns of as-prepared Co O /Co Se/Co Se (sample S4) with narrow scan (b) C 1s, (c) Co 2p, (d) Se 3d, and (e) O 1s.

0.85

have been evolved as splendid candidates for alkaline OER

because of their high abundance, low price, and elite redox

energy of selenium (Se) make the corresponding transition

23−25

metal selenides unique from other chalcogenides.

It is

property.

In recent times, extensive e�orts have been

reported that electrochemical superiority of a catalyst can be

boosted by hybridizing with another electrocatalyst. The

synergistic e�ect of the microstructural interface would play

the deliberate role to modulate the redox features via tuning

the electrochemical surface area as well as active site density for

a composite network. Among all the cobalt selenide

polymorphs, nonstoichiometric Co₀_.₈₅Se was widely employed

for electrochemical applications owing to its intrinsic

conductive nature, impressive capacity values, and layered

devoted to designing cobalt-containing non-oxide electro-

catalysts such as oxalate, nitride, phosphide, selenide, sul�de,

boride, etc. Liu et al. developed cobalt oxalate with a

controllable microrod and micropolyhedron-like architecture

for the alkaline OER purpose. Wu and co-workers designed

nanowire-like metallic Co N for the same. Direct growth of

Co P supported on Co foil was demonstrated as a bifunctional

catalyst for overall water splitting. Morphologically tuned

26,27

Co₀_.₈₅Se particularly for alkaline OER was reported by Li and

structural defects.

Jin et al. reported the direct growth of

co-workers. In another study, a CoS hollow nanosphere was

CoFe LDH/Co₀_.₈₅Se with a controllable nanosheet architec-

developed by an autoclaving method and showed 100%

ture on porous carbon cloth particularly for alkaline OER.

Faradaic e�ciency for overall water splitting. Amorphous

Self-supported electrocatalyst design has been captivated

tremendously due to mechanical integrity, seamless contact,

rapid charge transfer rate, and prudent active site distribution

Co B was also employed for the alike purpose. All these

reports clearly claim that development of nonprecious non-

oxide electrocatalysts is advantageous due to their prompt in

situ transformation to generate the active species for the

electrochemical water oxidation process.

over the substrate network. Xiong and co-workers prepared a

Co-foam supported Co@CoTe porous framework showing

excellent catalytic performance with long-term stability. Most

of the self-supported electrocatalysts have been prepared over

expensive nickel or cobalt foam. The aforementioned approach

has inspired us to design a self-supported electrocatalyst

without using costly foam. Furthermore, a porous Co O

Spinel-type Co O is well-known for their conductivity,

13,14

long-term stability, and easy handling features.

Electro-

chemical OER performance of Co O₄can be tuned by

numerous ways like morphological variation, quantum dot

design, controlling porosity, metal dopant, defect

microrod supported Co₀_.₈₅Se/Co Se electrocatalyst is rarely

9 8

generation, conductive substance incorporation, coupling

with other materials, and more. Metal ion dopants can

reported in the literature for alkaline OER application.

In the present communication, we have prepared a Co O

e�ectively modify the electronic state of Co O , which further

microrod supported Co₀_.₈₅Se/Co Se electrocatalyst and

elucidated the synergistic role of the Co O network for

3 4

9 8

validates improved activity with prompt OER kinetics via

lowering the charge-transfer resistance. Transition metal

selenides (TMSs) are exclusive for their extended metallicity.

In addition, the comparatively larger size and less ionization

OER in alkaline medium. For comparison, unsupported

Co₀_.₈₅Se/Co Se was directly prepared under the same

conditions and tested for OER. The synthesis strategy has

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

been optimized in such a way that exclusively retains the 1-D

microstructure of the Co O₄supported Co₀_.₈₅Se/Co Se

network in the presence of nanograins. Under similar reaction

for the as-prepared sample just before autoclaving treatment

(Figure S2). The aforementioned phase analysis clearly claims

that, initially, Co O was transformed into Co O /CoSeO ·

conditions, unsupported Co₀_.₈₅Se/Co Se delivers a �ake-like

nH O in a solution-phase reaction, followed by conversion into

architecture with numerous interparticle porosity. For the

alkaline OER process, the overpotential requirement is found

to be 400 and 330 mV for Co O and Co O /Co Se/Co Se ,

Co O /Co Se/Co Se under autoclave condition. For the

0.85

unsupported sample, the di�raction pattern resembles with

Co₀_.₈₅Se and Co Se (sample S5, Figure 1a-iii). No di�raction

0.85

respectively, to reach the benchmark 10 mA/cm _g_e_o. However,

unsupported Co₀_.₈₅Se/Co Se consumes 360 mV to reach the

peaks characteristic of elemental selenium (Se), �-Co(OH)₂,

or Co O have been perceived for the as-prepared unsupported

same. In addition, Co O /Co Se/Co Se possesses a low

sample. In alkaline medium, elemental Se undergoes

0.85

−

−2

Tafel slope, impressive stability, and a Faradaic e�ciency of

about �82%. The electrochemical mechanism has been

investigated with variation of pH and found to be primarily a

proton decoupled electron transfer (PDET) mechanism. The

superior performance of the supported electrocatalyst can be

disproportionation reaction to produce Se and SeO₃,

followed by the interaction with Co to form Co₀ _.₈ ₅ Se and

Co Se under autoclaving condition at 180 °C (Scheme

1−33

S1 ).

According to the Co-Se phase diagram, formation of

Co with octahedral coordination in Co₀_.₈₅Se is thermody-

namically favorable compared to Co Se where cobalt is

attributed to high double layer capacitance (C ), low charge-

transfer resistance, and exposure of additional active sites

present at the interlayer position. Postcatalytic analysis reveals

formation of agglomerated nanograins and a nano�ake-like

morphology, and scarcely any selenium (Se) has been

recovered from EDS and XPS study. Despite having higher

geometric performance for Co O /Co Se/Co Se , the

tetrahedrally coordinated in a lattice site. However,

reconstruction of the sublattice crystal structure from

Co₀_.₈₅Se to Co Se is kinetically labile because transformation

of the coordination site does not lead to an extensive energy

barrier for Co . We believe that Co at the tetrahedral site in

spinel Co O facilitates the reconstruction of the sublattice via

0.85

intrinsic activity is quite inferior compared to unsupported

Co₀_.₈₅Se/Co Se , indicating the contribution of abundant

reducing the energy gap during selenization that further

appears as prominent Co Se along with Co Se for the

0.85

active site density toward improved geometrical activity. This

supported catalyst under autoclaving condition. The

structural characteristics were further studied by FTIR analysis

(Figure S3 ). The as-prepared sample shows a broad peak

work clearly authenticates the in�uence of the Co O support

to modulate the alkaline OER performance of the Co₀_.₈₅Se/

Co Se network by improving the exposure of active sites.

−

−1

region at 1638 cm along with a peak-pair around 1312 cm

(

−

symmetric) and 1378 cm (asymmetric) representing the

. RESULTS AND DISCUSSION

carbonyl vibrational mode for the oxalate group. In addition,

−1

peaks appearing around 827 and 498 cm manifest the O-C-O

asymmetric vibration mode and metal-oxygen vibration mode,

respectively. The broad peak around 3378 cm⁻visibly

First, a Co O microrod was synthesized by a ligand assisted

route using cobalt nitrate hexahydrate and oxalic acid at room

temperature in a water-ethanol medium, followed by

calcination at 350 °C. The as-prepared Co O was further

indicates the presence of structural water of CoC

O ·nH O

4 2

selenized under autoclaving condition at 180 °C for 12 h and

washed with ethanol, followed by drying at room temperature

(Figure S3-i ). However, the annealed sample displays vibra-

−

tional bands around 566 and 667 cm , indicating the Co-O

see details in the Experimental Section, Supporting

stretching vibrational mode particularly for Co (coordinated

Information). As shown in Figure S1, the XRD di�raction

peaks around 2� � 18.97°, 22.84°, 30.38°, 35.14°, and 47.24°

can be designated as (202), (004), (400), (022), and (602)

planes, respectively, for CoC O ·nH O (sample S1, crystal

at octahedral site of the spinel) and Co (coordinated at

tetrahedral site of the spinel), respectively (Figure S3-ii).

This clearly supports the formation of Co , which is

previously evidenced from XRD study. For the selenized

sample, the vibrational spectrum shows weak peaks around 728

system: orthorhombic, JCPDS File # 25-0250). After

calcination treatment, the di�raction peaks at 2� � 18.9°,

−

cm , indicating the Co-Se vibration mode along with a

distinct Co vibrational band. Furthermore, weaker peaks

1.24°, 36.94°, 38.61°, 44.74°, 55.74°, 59.54°, and 65.4° can

3 4

−

be matched with (111), (220), (311), (222), (400), (422),

511), and (440) planes, respectively, for Co O (sample S2,

positioned around 1010 and 1347 cm represent O-H

bending (in-plane) and stretching modes, respectively, for

absorbed water molecules of the cobalt selenite complex

(

Figure 1a-i, crystal system: cubic, JCPDS File # 43-1003).

Selenization treatment further endows the appearance of

Co₀_.₈₅Se and Co Se along with Co O for the as-prepared

37,38

(Figure S3-iii ).

After autoclave reaction, the intensity of

−

the vibrational band representing Co-Se around 714 cm is

sample (sample S4, Figure 1a-ii). XRD signals observed at 2�

33.19°, 44.81°, 50.65°, 59.98°, 61.86°, 69.9°, and 71.35° can

signi�cantly increased. The sample also retains the vibrational

−

�

bands at 1010 and 1347 cm . However, the observed Co-O

−

be resembled with (101), (102), (110), (103), (112), (202),

and (004) planes, respectively, for Co₀_.₈₅Se (crystal system:

hexagonal, JCPDS File # 52-1008). In addition, di�raction

peaks at 2�� 28.36°, 29.68°, 49.36°, and 58.66° can be labeled

to (311), (222), (440), and (622) planes, respectively, for

Co Se (crystal system: cubic, JCPDS File # 01-089-4180). To

vibrational peaks at 496 and 1608 cm clearly suggest the

presence of a Co O structural unit within the Co O /

Co₀_.₈₅Se/Co Se composite network (Figure S3-iv). For the

unsupported sample, the spectrum retains an intense vibra-

−

tional band at 714 cm displaying strong Co-Se interaction.

However, for template-free synthesis (i.e., unsupported

sample), hardly any Co-O interaction is noted, revealing the

absence of a Co O structural unit within the Co Se/Co Se

understand the autoclaving e�ect during transformation from

Co O to the Co O /Co Se/Co Se composite network,

0.85

phase analysis was performed for the as-prepared sample

before autoclaving treatment. The corresponding XRD pattern

reveals the appearance of CoSeO ·nH O (sample S3, crystal

composite network (Figure S3-v). The elemental state in the

Co O /Co Se/Co Se composite network was further

studied by using XPS. The C 1s XPS spectrum shows peak

positions at 284.2 and 291.1 eV, indicating C-C and C-F

0.85

system: monoclinic, JCPDS File # 25-0125) along with Co O

Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 2. FESEM images of as-prepared samples of (a) CoC O ·2H O (sample S1), (b) Co O (sample S2), (c) Co O /Co Se/Co Se (sample

0.85

S4), and (d) Co₀_.₈₅Se/Co Se (sample S5).

interactions, respectively. The observed peak for C-F is due to

usage of a Na�on polymer as binder for the carbon paper

supported dropcasted electrode (Figure 1b). As displayed in

the Co 2p narrow scan, the appearance of two distinct peaks at

surface (Figure 2b). The microstructural change could be

addressed due to nonequilibrium heat treatment at high

temperature during annealing of the as-prepared CoC O ·

2H O sample. After selenization treatment, agglomerated

81.5 eV (Co 2p₃_/₂) and 797.5 eV (Co 2p₁_/₂) with the

nanograins along with a grainy rodlike framework is

encountered for Co O /Co Se/Co Se (Figure 2c). Close

separation in binding energy (Co 2p₁− Co 2p₃_/₂) around

0.85

6.0 eV validates the existence of Co within the composite

inspection further reveals a protrusion of nanograins over the

surface of the 1-D microrod throughout the sample (Figure

S4 ). For unsupported Co₀_.₈₅Se/Co Se , the FESEM image

network. The binding energy values at 779.5 eV (Co 2p₃

)

and 795.8 eV (Co 2p₁_/₂) are ascribed to the presence of Co .

In addition, the satellite peak at 785.8 eV clearly authenticates

displays a spongy �ake-like morphology (Figure 2d). For

further details of the microstructural appearance, TEM study

was performed for all the samples. The CoC O ·2H O sample

the coexistence of both Co²and Co , which is previously

reported by Frost and co-workers (Figure 1c). The binding

energy values around 54.1 and 59.7 eV appear due to Se 3d

and SeO , respectively. Furthermore, deconvolution of the Se

shows a microrod-like architecture with a diameter of about

60−200 nm and interplanar spacing of 0.25 nm representing

the (022) plane from HRTEM study (Figure S5). Elemental

mapping study with EDS shows a uniform distribution of C,

Co, and O throughout the microrod architecture of CoC O ·

d peak manifests the binding energy values at 53.6 and 54.2

eV representing Se 3d and Se 3d₃_/₂, respectively (Figure

1,42

d).

For the O 1s spectrum, peaks at 530.9, 532.08, and

34.6 eV can be attributed to lattice oxygen, a surface adsorbed

2H O (Figure S6). For Co O , the microrod-like architecture

hydroxyl group, and structural water molecules, respectively

is retained along with interparticle porosity and long-range

connectivity. However, pores are not uniform all over the oxide

framework of the sample. The high-resolution image also

discloses a stone-wall-like assembly which is formed from

random particle aggregation maintaining a 1-D orientation

(Figure S7). Elemental mapping and EDS study further show

the distribution of Co and O all over the porous network

(

Figure 1e).

Microstructural characterization of samples was performed

by FESEM study (Figure 2). For the as-prepared CoC O ·

H O sample, a uniform microrod-like architecture is obtained

along with a smooth surface (Figure 2a). Nevertheless, Co O

displays a porous network of 1-D microstructure with a rough

Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 3. (a−d) TEM images, (e−h) elemental mapping, and (i) HRTEM image of grainy rodlike morphology of as-prepared Co O /Co₀_.₈₅Se/

Co Se (sample S4). (j) TEM image of agglomerated grainy network-like morphology, (k) HRTEM image from agglomerated nanograins, and (l,

m) EDS study of Co O /Co Se/Co Se (sample S4).

0.85

Figure S8). For the Co O supported selenized sample, the

ture during the selenization process, which is further

transformed into the Co O /Co Se/Co Se grainy network

coexistence of both nanograins and a grainy rod architecture is

clearly perceived in the TEM investigation (Figure 3a). Close

inspection reveals that the formation of nanograins took place

from the edge of the porous rodlike architecture (Figure 3b,c).

These nanograins are irregular in size and shape and

aggregated in a random manner all over the network (Figure

0.85

under autoclaving condition. Additional TEM study is also

carried out to scrutinize the role of Co O as a self-supported

framework. For the unsupported Co₀_.₈₅Se/Co Se sample, the

TEM image shows a spongy �ake-like microstructure with

numerous interparticle porosity (Figure 4a,b). Additionally,

close inspection reveals that random alignment of �akes

generates a crumpled thin-layer sheetlike architecture (Figure

4c). HRTEM study renders an interplanar separation of about

0.20 nm manifesting the (102) plane of the Co₀_.₈₅Se sample

(Figure 4d). Elemental mapping using EDS analysis further

indicates a uniform distribution of Co and Se all over the

network (Figure 4e−h). Microstructural study fairly concludes

the in�uence of a Co O microrod as a self-supported cobalt

d). Additionally, uniform distribution of Co, Se, and O

appeared from the elemental mapping study (Figure 3e−h).

The HRTEM image shows lattice fringe with 0.20 and 0.24 nm

representing the (102) and (311) planes for Co₀_.₈₅Se and

Co O , respectively, at the interface of the grainy network

(Figure 3i). For further con�rmation, HRTEM study was also

performed from the agglomerated network of the nanograin-

like morphology (Figure 3j). From HRTEM analysis,

interplanar distance appears around 0.20 nm, indicating the

presence of Co₀_.₈₅Se for agglomerated nanograins (Figure 3k).

EDS study further con�rms the existence of Co, Se, and O

throughout the composite sample (Figure 3l,m). However,

morphological analysis is also investigated for the as-prepared

sample before autoclaving treatment (Figure S9). The FESEM

image represents a porous rodlike morphology along with

aggregated particles throughout the framework (Figure S9a).

TEM study displays a porous architecture with a stone-wall-

like assembly with interplanar spacing around 0.30 nm

manifesting the (012) plane of CoSeO ·nH O (Figure S9b −

precursor toward the formation of a grainy network of the as-

prepared selenized sample.

Electrochemical activity for the alkaline oxygen evolution

reaction (OER) was checked for all the carbon paper

dropcasted samples using the iR-corrected backward LSV

technique (Figure 5a). Among all the samples, Co O /

Co₀_.₈₅Se/Co Se shows superior performance with a 330 mV

overpotential for a benchmark current density of 10 mA/

cm _g_e_o, whereas 390, 400, and 440 mV are the required

overpotential values for CoC O ·2H O, Co O , and Co O /

CoSeO ·nH O, respectively. To garner the role of Co O ,

e ). This clearly validates the transformation trajectory from

unsupported Co₀_.₈₅Se/Co Se was also examined for electro-

9 8

Co O microrod to a Co O /CoSeO ·nH O porous architec-

chemical water oxidation reaction in alkaline medium.

Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 4. (a −c) TEM images, (d) HRTEM image, (e −g) elemental mapping, and (h) EDS study of unsupported Co ₀ _.₈ ₅ Se/Co Se (sample S5).

Figure 5. (a) iR-corrected backward LSV scan of di�erent electrocatalysts, (b) overpotential value to reach benchmark 10 mA/cm²_g_e_ofor di�erent

electrocatalysts, (c) Tafel slope analysis of di�erent electrocatalysts, (d) Nyquist plots of di�erent electrocatalysts, (e) backward LSV scan and (f)

linear plot of current density of Co O /Co Se/Co Se (sample S4) with variation of pH.

0.85

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Co₀_.₈₅Se/Co Se exhibits a 360 mV overpotential value to

Co O to accelerate charge transport features and conductivity

3 4

reach the same 10 mA/cm

(Figure 5b). To eliminate any

of the selenized network. The experiment was also carried out

in di�erent pH mediums to investigate the mechanistic

pathway of alkaline OER. The following mathematical relation

can be used to calculate the proton reaction order (�RHE) for

alkaline OER in the presence of the Co O /Co Se/Co Se

geo

uncertainty in the overpotential values, we have checked the

accuracy and averaged the overpotential values along with the

calculation of standard deviations and marginal errors for

Co O /Co Se/Co Se and Co Se/Co Se samples (Fig-

0.85

ures S10 and S11 , Table S1 ). These results clearly validate the

importance of Co O microrods as a self-supported cobalt

composite network

i � log(j)y

i � E y i � E

z j

precursor to improve the water oxidation process. However,

the unsupported catalyst also shows improved catalytic activity

after calcination under an inert atmosphere with the distinct

phase transition from Co₀_.₈₅Se/Co Se to CoSe (Figure S12).

z j

�_R_H_E= j

z = −j

z /j

z j

� pH

� log(j)

{

where �RHE is determined in RHE scale. Figure 5e shows

consecutive increments in current density with increasing pH

from 12 to 14. Furthermore, the �RHE value is estimated as

For further con�rmation, CV measurements were performed

with uncompensated solution resistance (R ) correction in

Figure S13 . Just before checking the activity of Co O /

0.65 and 0.94 for Co

Co Se , respectively, indicating a proton decoupled electron

transfer (PDET) assisted electrochemical water oxidation

/Co0.85Se/Co

and Co0.85Se/

Co₀_.₈₅Se/Co Se , precondition of the working electrode was

9 8

performed within 1.0−1.65 V at 50 mV/s until reproducible

CV data were obtained in alkaline medium. In the �rst cycle of

electrochemical precondition, a conspicuous redox wave is

obtained, indicating the signature electrochemical features of

the precatalyst while the intensity of the redox wave (oxidation

peaks) is signi�cantly abridged in the next cycle (Figure

mechanism (Figures 5f and S16). The mathematical

expression of current for alkaline OER can be presented as

−

−�G/RT

j = [OH ].�.e

�

refers to surface coverage which is predominantly comprised

−

S14). Such transformation in the voltammogram manifests

that the precatalyst is irreversibly transformed into species with

higher oxidation states just after the �rst cycle. However, the

conspicuous redox wave (reduction peaks) progressively

appeared with enhanced intensity, suggesting continuous

formation of oxidized cobalt species up to 20 cycles of

precondition to achieve the steady state. During the cycling of

cobalt selenide, two oxidation peaks were observed at 1.19 and

with *OH or *OOH sites, and [OH ] stands for the

concentration of electrolyte. Thus, several factors like surface

coverage (�), pH of solution (concentration of [OH ]), and

exponential parameter of adsorbed species are responsible for

the sharp upsurge in current density with increasing pH of

electrolyte medium. The �RHE value elucidates the mecha-

nistic trajectory as OOH (ads) + OH � OO (ads) + H + e ,

thereby indicating the prudent role of electrolyte concentration

−

49,50

.48 V, indicating Co /Co and Co /Co oxidation

processes, respectively, for the corresponding catalytic network

Figure S15 ). The appearance of CoOOH can be assigned by

the initial redox peak position, which is further turned into

to drive the reaction via a decoupling mechanism.

investigate the role of oxidized cobalt species, a �ve-step

mechanistic pathway was proposed by Qiao and co-workers.

As shown in Scheme S3, a third step was recommended as the

rate-determining step (RDS) which involves an oxyhydroxide

(−OOH) intermediate. It was hypothesized that higher charge

CoO₂representing formation of a Co site during the

5,46

electrochemical process (Scheme S2 ).

Interestingly, a

catalytic center comprising Co⁴has been already mentioned

as the primary active site for the alkaline OER mechanism. The

distinct reduction peaks at 1.11 and 1.44 V are also

encountered for the same. In addition, higher area under the

CV curve suggests that the Co O /Co Se/Co Se network

−

on Co species in the precatalyst electrostatically helps [OH ]

adsorption, enabling the formation of the (−OOH)

intermediate, followed by deprotonation to yield O . In

addition, electrochemical transformation from Co to Co

is also involved during electrocatalysis that expedites the

0.85

possesses a higher number of active sites compared to

Co₀_.₈₅Se/Co Se . The as-displayed CV plot also represents

deprotonation of the oxyhydroxide (−OOH) intermediate to

produce O in alkaline medium. We also believe that Co

that incorporation of Co O indeed facilitates the Co /Co

centers in Co O facilitate the formation of the oxyhydroxide

3 4

transition and reducibility of the catalytic framework which

further attributes higher active site density for the electro-

catalyst in alkaline condition. The Tafel plot is further

examined to interpret the kinetics of the alkaline OER process

(−OOH) intermediate, thereby reducing the energy gap for

2+ 3+ 3+ 4+

Co /Co and Co /Co transition favoring Co O /

Co₀_.₈₅Se/Co Se as a better electrocatalyst compared to

9 8

Co₀_.₈₅Se/Co Se . Interestingly, the geometrical current density

(

Figure 5c). Co O /Co Se/Co Se renders the Tafel slope

value is found to be approximately 2 times higher for Co O /

0.85

3 4

value of about 91 mV/dec, which is lower in comparison with

Co₀_.₈₅Se/Co Se compared to Co₀_.₈₅Se/Co Se in 1 (M)

9 8 9 8

that of CoC O ·2H O (93 mV/dec), Co O (118 mV/dec),

KOH, revealing the importance of the Co O support which

3 4

and Co O /CoSeO ·nH O (134 mV/dec). Interestingly, the

exposes a higher number of active centers and interlayer active

Tafel slope value for the composite network is even lower than

that for unsupported Co₀_.₈₅Se/Co Se (98 mV/dec), implying

sites that further contribute in surface coverage (�) for alkaline

OER. The results also authenticate the bene�t of Co as

rapid charge transfer kinetics due to the presence of Co O

within the composite network (Table S2). EIS (electro-

chemical impedance spectroscopy) measurements were further

Co O in the precatalyst for the alkaline OER mechanism.

Electrochemical surface area (ECSA) measurement was

studied to further illustrate the role of Co O within the

performed to measure charge-transfer resistance (R ) of the

selenized network. Recently, Surendranath et al. have reported

a precise analytical method for double layer capacitance (C_d_l)

calculation which is further employed to estimate the ECSA of

electrocatalyst in alkaline medium (Figure 5d). The R value is

.687 ohm for Co O /Co Se/Co Se , which is lower than

3 4 0.85 9 8

that for CoC O ·2H O (4.199 ohm), Co O (10.81 ohm),

a catalyst. For Co O /Co Se/Co Se , the C value is about

3 4 0.85 9 8 dl

Co O /CoSeO ·nH O (30.08 ohm), and Co Se/Co Se

8.19 mF/cm , which is relatively higher than that for Co₀_.₈₅Se/

0.85

(

3.698 ohm). The above study fairly endows the bene�t of

Co Se (1.08 mF/cm ), thereby indicating higher active site

Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 6. CV curves recorded at di�erent scan rates to estimate double layer capacitance (C ) for as-prepared samples of (a) Co O /Co₀_.₈₅Se/

Co Se (sample S4) and (b) Co Se/Co Se (sample S5). (c) Linear �t for estimation of double layer capacitance (C ) from DLC current vs scan

0.85

rate plot. CV curves recorded at di�erent scan rates to estimate active site density for as-prepared samples of (d) Co O /Co Se/Co Se (sample

0.85

1/2

S4) and (e) Co₀_.₈₅Se/Co Se (sample S5). (f) Linear �t for estimation of active sites from Co /Co peak current vs scan rate plot.

density in the presence of the self-supported Co O framework

S_g_e_o

TOF = j ₄

(

Figure 6a−c). For comparison, the C_d_lvalue was also

calculated for CoC O ·2H O, Co O , and Co O /CoSeO ·

^H^e^r^e^,^F^aⁿ^d^Sgeo stand for Faraday constant (96 485 C/mol)

and surface area (geometric area of substrate), respectively.

nH O samples (Figure S17). In addition, polarization curves of

the electrocatalysts were normalized by their corresponding

However, concentration of active species (m) can be estimated

based on the position of accessible active sites in the catalytic

matrix. To incorporate the geometric area value in TOF

relation, substrate scope analysis was also conducted for the

best catalyst over glassy carbon and porous carbon paper

surfaces. Carbon paper dropcasted Co O /Co Se/Co Se

ECSA to understand the per site catalytic activity. The

obtained results clearly validate that unsupported Co₀_.₈₅Se/

Co Se exhibits high per site catalytic activity compared to

Co O /Co Se/Co Se , whereas the per site activity is almost

0.85

identical for both CoC O ·2H O and Co O (Figure S18).

0.85

Despite having higher intrinsic e�ciency for unsupported

catalyst delivers greater mass activity compared to glassy

carbon, conveying the advantage of carbon paper as a substrate

due to its inherent porous network (Figure S20). The

improved e�ciency of the carbon paper dropcasted electro-

catalyst may be originated due to spontaneous electrolyte

penetration that exposes more active sites from the surface as

well as the bulk of the catalytic framework. For the carbon

paper dropcasted electrocatalyst, charge storage mechanism

and position of active sites can be predicted by the following

equations:

Co₀_.₈₅Se/Co Se , geometrical OER performance is superior for

Co O supported Co₀_.₈₅Se/Co Se₈catalyst because of

exposure of the higher number of active centers from the

surface as well as bulk. For further clarity, C measurement was

also performed before precondition in alkaline medium. The

as-prepared electrocatalyst shows a C value of about 605.7

and 105.64 �F/cm for Co O /Co Se/Co Se and Co₀_.₈₅Se/

0.85

Co Se , respectively, before electrochemical precondition

Figure S19 ). Such increments in C value could be attributed

to the formation of *OH or *OOH sites that further ensures

change in surface coverage (�) during precondition of the

precatalyst in alkaline medium. The change in C_d_lvalue is

much higher in the presence of the Co O framework that may

= a�

log i = log a + b log �

drive to form a higher number of accessible active sites in

alkaline condition. It clearly authenticates the importance of

the Co O precursor within the selenized network for

improvement of geometrical OER activity. To elucidate further

the electrochemical performance, intrinsic activity was

compared for both the samples. Intrinsic activity of the sample

is represented by TOF (turn over frequency) using the

following mathematical equation:

“

b” is estimated from the log i vs log �plot, where i and �stand

for current and scan rate, respectively. Calculation of the “b”

parameter endows the guidance about location of active sites

within the catalytic framework. For b � 1, the surface

controlled process predominates, while the di�usion process

via ion intercalation is primarily responsible for b � 0.5. CV

measurements at di�erent scan rates were performed for both

Co O /Co Se/Co Se and Co Se/Co Se samples, and

0.85

Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 7. (a−c) FESEM and EDS images, (d−f) TEM images, (g) HRTEM image, and (h−k) elemental mapping of postcatalytic Co O /

Co Se/Co Se (sample S4).

0.85

0.0076 and 0.0102 s⁻¹for Co O /Co Se/Co Se and

3 4 0.85 9 8

the “b” value was obtained from Co /Co oxidation current

Figure 6d,e). The calculated “b” is found to be 0.67 and 0.71

for Co O /Co Se/Co Se and Co Se/Co Se , respectively,

(

Co₀_.₈₅Se/Co Se , respectively, at � = 400 mV. Despite having

9 8

a higher intrinsic activity (TOF) for Co₀_.₈₅Se/Co Se ,

0.85

manifesting the existence of additional active centers at

interlayer positions in the presence of Co O (Figure S21).

The CV plot further demonstrates a quasi-reversible process

geometrical activity is much superior for Co O /Co₀_.₈₅Se/

3 4

Co Se . It indeed validates that higher geometrical activity of

Co O /Co Se/Co Se could be obtained due to presence of

3 4 0.85 9 8

where separation potential (�E ) is found to increase with

Co O which exposes a higher number of active centers and

3 4

increasing the scan rate value. Thus, active site concentration

interlayer active sites that further contributes in surface

(

m) can be calculated for the quasi-reversible process using the

coverage (�) for alkaline OER. The above results also reveal

i vs v plot:

the importance of the self-supported Co O network for the

selenization process to acquire a splendid OER electrocatalyst

in alkaline medium.

nFDv

i = ±0.436nFA_r_e_a_l

The Co O /Co Se/Co Se composite network is further

0.85

subjected for phase and microstructural analysis after electro-

chemical precondition. In Figure S22a, the XRD di�raction

Considering the same di�usion constant (D) value for both

Co O /Co Se/Co Se and Co Se/Co Se samples, active

0.85

site concentration (A C) is estimated using D = 1 and n = 1,

patterns of the Co O /Co0.85Se/Co Se network before and

3 4 9 8

real

where A_r_e_a_land C stand for electrochemical area and per site

after electrochemical precondition are shown. For further

clarity, structural and phase analyses were also performed at

successive preconditioning cycles (Figure S22b). Before

electrochemical precondition, the carbon paper supported as-

prepared precatalyst shows signature di�raction peaks for

,58

concentration, respectively.

The calculated concentrations

of active species are 3.078 × 10⁻and 1.796 × 10 mol for

−6

Co O /Co Se/Co Se₈and Co₀_.₈₅Se/Co Se , respectively

0.85

(

Figure 6f). Nevertheless, the obtained TOF values are

Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 8. (a) Stability study with inset showing CV scan before and after stability test, (b) iR-corrected backward LSV scan before and after

stability test with inset showing overpotential value for 10 mA/cm _g_e_o, (c) Nyquist plots before and after stability test, and (d) analysis of Faradaic

e�ciency for Co O /Co Se/Co Se (sample S4).

0.85

carbon paper (2� � 18.09°, 26.55°, 54.45°), Co O (2� �

formation of CoOOH from the electrocatalyst. It is worthy to

8.9°, 36.94°, 38.61°), Co₀Se (2� � 33.19°, 44.81°, 50.65°),

state that no signi�cant change is encountered from the 1st to

the 20th cycle during electrochemical precondition that indeed

suggests evolution of an active phase in initial segments of the

catalysis in alkaline medium. To garner the elemental state of

the postcatalytic sample, XPS measurement was also examined

(Figure S24 ). Binding energy peaks from the C 1s plot around

284.6, 286.2, and 287.8 eV stand for the carbon paper. From

the narrow scan of the Co 2p spectrum, the appearance of

peaks at 780.4 (Co 2p₃_/₂) and 795.3 eV (Co 2p₁_/₂) implies the

existence of a Co state with (Co 2p₁_/₂− Co 2p₃_/₂)

separation of about 15.1 eV. In addition, satellite peaks at

783.04 and 798.5 eV corroborate the same. Interestingly,

hardly any distinct binding energy value is obtained from the

Se 3d spectrum, revealing the absence of selenium species in

the postcatalytic sample. Hu et al. have proposed in situ

.85

and Co Se (2� � 29.68°, 49.36°). After the �rst cycle of

precondition, hardly any di�raction peaks from Co O and

Co₀_.₈₅Se are detected, while the peaks from Co Se are

retained. However, no signi�cant change in elemental phase is

observed from the �rst cycle to the 20th cycle during

electrochemical precondition. These observations evidently

claim the fact that electrochemical transformation into

CoOOH is most likely occurring from the �rst cycle of

precondition during electrolysis. For further con�rmation,

Raman measurement was performed to scrutinize the electro-

chemical transformation of the precatalyst during alkaline OER

Figure S23). The appearance of peaks positioned at 187.9,

54.2, 540.2, and 668.5 cm⁻can be resembled with A , E , F ,

g g 2g

and A , respectively, indicating the presence of Co₀_.₈₅Se and

−2

Co Se for the as-prepared sample. The signature peaks in

transformation of selenide to SeO₃/SeO₄that undergoes

dissolution in 1 (M) KOH solution during electrocatalysis.

We believe that the selenide species etches out as K SeO /

−

the low frequency region at 187.9 and 258.5 cm stand for

bending and stretching modes of Se-Se, respectively. After

the �rst cycle of electrochemical precondition, a conspicuous

sharp peak appears at 462.7, 507.6, and 670.5 cm , divulging

K SeO with the formation of CoOOH during alkaline OER at

2 4

−

64,65

pH = 14.

The etching of selenide was further evidenced by

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

The Supporting Information is available free of charge at

Article

microstructural analysis and EDS study. FESEM images show

the coexistence of particle and petal-like morphology

throughout the postcatalytic network (Figure 7a,b). However,

signi�cant reduction in Se counts is conceived from EDS study

that further supports previous XPS analysis for Co O /

3. CONCLUSION

Co O supported Co Se/Co Se was synthesized by a two-

0.85

step solution-based technique. In the �rst step, a Co O

microrod was prepared by an oxalic acid assisted wet chemical

process, followed by calcination at 350 °C. Thereafter, Co O

^C^o⁰.85Se/Co Se after electrochemical precondition (Figure

microrods were selenized by an autoclaving method at 180 °C

for 12 h. Phase investigation shows the formation of Co₀_.₈₅Se/

Co Se in the presence of Co O , which is further supported

c). The appearance of K in EDS further authenticates the

elemental etching process during the alkaline OER mechanism.

TEM investigation indeed renders agglomerated particles as

well as a petal-like architecture along with long-range pore

connectivity (Figure 7d−f). HRTEM analysis depicts an

interplanar spacing of about 0.220 nm paving the presence

of the (006) plane of CoOOH (Figure 7g, crystal system:

by XPS analysis. Morphological study reveals the coexistence

of nanograins and a grainy rodlike architecture throughout the

network. It also discloses the protrusion of nanograins from the

surface of Co O which is further supported by HRTEM

analysis. Interestingly, Co O /Co Se/Co Se reaches the

benchmark current density of 10 mA/cm

0.85

rhombohedral, JCPDS File # 014-0673). Elemental mapping

with the need of

geo

further endows a uniform distribution of K, Co, and O all over

the postcatalytic framework (Figure 7h−k). Furthermore,

morphological evolution of the precatalyst was also studied

at consecutive preconditioning cycles, indicating microstruc-

tural reformation is most likely occurring from the 1st cycle of

precondition during electrolysis (Figure S25). Morphological

study using FESEM and TEM clearly discloses the abrupt

change in architecture after OER in alkaline medium. The

change in the morphology into a petal-like network exposes the

bulk of the material to the electrolyte, and thereby the Se

species present in the bulk etches away along with the surface

species.

an overpotential around 330 mV, whereas unsupported

Co₀_.₈₅Se/Co Se consumes 360 mV for the same, indicating

the advantage of Co O microrods as the self-supported cobalt

precursor. The improvement in catalytic property can be

attributed on the basis of a change in ECSA (C ) value,

interlayer active sites (b value), and active center density for

the alkaline OER. Despite having the higher geometrical

activity, intrinsic performance is inferior for the Co O /

Co₀_.₈₅Se/Co Se network compared to unsupported Co₀_.₈₅Se/

Co Se . Electrochemical study clearly demonstrates that the

higher number of active centers could be the reason for

extraordinary geometrical OER performance which is achieved

in the presence of self-supported Co O microrods that further

For bulk-scale application of the electrocatalyst, the Co O /

^C^o⁰.85Se/Co Se network was further tested using a multistep

illustrates long-term stability, mechanical strength in di�erent

potential windows, and rapid charge transport properties. This

study indeed addresses the utility of a metal oxide precursor

with a controllable morphology in comparison with a common

metal source to design the electrocatalytic framework for

alkaline OER application. We also believe that the precursor

modi�cation approach can be employed for other self-

supported transition metal complexes regarding energy and

environmental applications.

chronopotentiometric method. It shows moderate stability at

di�erent potential ranges including 1.55 V for 15 min, thereby

indicating excellent conductivity, charge transfer kinetics, and

mechanical strength of electrode materials (Figure S26 ). The

carbon paper dropcasted best catalyst was further examined by

potentiodynamic cycling to ensure the stability issue. To

attain the benchmark 10 mA/cm²geo^,^o^v^e^r^p^o^t^eⁿ^tⁱ^a^l^v^a^l^u^e^s^a^r^e

found to be 335, 340, and 360 mV after 10, 50, and 100 cycles,

respectively, indicating only a slight decay in electrochemical

performance after 100 cycles (Figure S27). It clearly represents

that electrochemical features of Co O /Co Se/Co Se are

ASSOCIATED CONTENT

�

0.85

sı Supporting Information

almost retained even after 100 cycles. A chronoamperometry

technique was also performed to check 24 h stability test.

Figure 8a shows near-identical activity even after 24 h usage

which is further supported by CV analysis of the catalyst after

the chronoamperometry experiment. The backward LSV scan

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02618.

XRD and FTIR patterns, FESEM and TEM images,

elemental mapping and EDS, cyclic voltammograms

(

CV), corrected LSV patterns using the iR-compensa-

(iR-corrected) manifests that the performance of the catalyst is

tion technique, double layer capacitance (C ) measure-

slightly declined after the stability test. Thus, the postcatalytic

ment, comparative study with glassy carbon electrode,

calculation of b value from scan rate and peak current

plot, XRD and XPS analysis of postcatalytic sample, and

multistep chronopotentiometric technique analysis

Co O /Co Se/Co Se network achieves 10 mA/cm at

geo

0.85

.575 V vs RHE that requires 15 mV more overpotential after

long-term usage (Figure 8b). After stability test, charge-transfer

resistance (R ) is found to be 2.794 ohm, which is only slightly

(PDF )

higher than that for the as-prepared catalyst (2.687 ohm). It

indeed conveys near-identical charge transfer kinetics and

catalytic activity without any loss of mechanical stability

AUTHOR INFORMATION

�

(

Figure 8c). To scrutinize the ultimate electrochemical

Corresponding Authors

performance, Faradaic e�ciency was also measured for the

best sample, rendering �82% e�ciency for OER application in

alkaline medium (Figure 8d). Catalytic performance of our

Sourav Ghosh − Department of Chemical Sciences, Indian

Institute of Science Education and Research (IISER) Kolkata,

Mohanpur, West Bengal 741246, India ; Phone: +91(0)

9007603474; Email: sourav.g1989@gmail.com ; Fax: 91-

33-25873020

best sample is also compared with the standard RuO and

earlier reported cobalt-based electrocatalyst representing akin

activity in terms of iR-corrected backward LSV scan (Table

Venkataramanan Mahalingam − Department of Chemical

Sciences, Indian Institute of Science Education and Research

(IISER) Kolkata, Mohanpur, West Bengal 741246, India;

orcid.org/0000-0003-1414-805X; Phone: +91(0)

7−85

S2 ).

This result also forti�es the bulk scale implementa-

tion of the Co O /Co Se/Co Se framework as a commer-

0.85

cial electrocatalyst for alkaline water oxidation purpose.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Zhao, S.-J.; Jiang, N.; Xu, A.-W. Direct Growth of Cobalt-Rich Cobalt

Article

007603474; Email: mvenkataramanan@yahoo.com;

(9) Yuan, C.-Z.; Zhong, S.-L.; Jiang, Y.-F.; Yang, Z. K.; Zhao, Z.-W.;

Phosphide Catalysts on Cobalt Foil: An Efficient and Self-supported

Bifunctional Electrode for Overall Water Splitting in Alkaline Media.

J. Mater. Chem. A 2017, 5, 10561−10566.

Fax: 91-33-25873020

Authors

Gouri Tudu − Department of Chemical Sciences, Indian

Institute of Science Education and Research (IISER) Kolkata,

Mohanpur, West Bengal 741246, India

Ayan Mondal − Department of Chemical Sciences, Indian

Institute of Science Education and Research (IISER) Kolkata,

Mohanpur, West Bengal 741246, India

Sagar Ganguli − Department of Chemical Sciences, Indian

Institute of Science Education and Research (IISER)

Kolkata, Mohanpur, West Bengal 741246, India;

orcid.org/0000-0002-1209-1689

Harish Reddy Inta − Department of Chemical Sciences, Indian

Institute of Science Education and Research (IISER)

Kolkata, Mohanpur, West Bengal 741246, India

(10) Zhong, Q.-S.; Xia, W.-Y.; Liu, B.-C.; Xu, C.-W.; Li, N. Co 0.85 ^S ^e

on Three-Dimensional Hierarchical Porous Graphene-like Carbon for

Highly Effective Oxygen Evolution Reaction. Int. J. Hydrogen Energy

11) Ma, X.; Zhang, W.; Deng, Y.; Zhong, C.; Hu, W.; Han, X. Phase

019, 44, 10182−10189.

and Composition Controlled Synthesis of Cobalt Sulfide Hollow

Nanospheres for Electrocatalytic Water Splitting. Nanoscale 2018, 10,

816−4824.

(12) Masa, J.; Weide, P.; Peeters, D.; Sinev, I.; Xia, W.; Sun, Z.;

Somsen, C.; Muhler, M.; Schuhmann, W. Amorphous Cobalt Boride

(Co B) as a Highly Efficient Nonprecious Catalyst for Electro-

chemical Water Splitting: Oxygen and Hydrogen Evolution. Adv.

Energy Mater. 2016, 6, 1502313.

(13) Zhao, Q.; Yan, Z.; Chen, C.; Chen, J. Spinels: Controlled

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c02618

Preparation, Oxygen Reduction/Evolution Reaction Application, and

Beyond. Chem. Rev. 2017, 117, 10121−10211.

(14) Zhang, S. L.; Guan, B. Y.; Lu, X. F.; Xi, S.; Du, Y.; Lou, X. W.

Author Contributions

G.T. and A.M. contributed equally.

D. Metal Atom-Doped Co O Hierarchical Nanoplates for Electro-

3 4

†

catalytic Oxygen Evolution. Adv. Mater. 2020, 32, 2002235.

15) Sidhureddy, B.; Dondapati, J. S.; Chen, A. Shape-Controlled

Synthesis of Co O for Enhanced Electrocatalysis of the Oxygen

16) Zhang, G.; Yang, J.; Wang, H.; Chen, H.; Yang, J.; Pan, F.

Notes

The authors declare no competing �nancial interest.

Evolution Reaction. Chem. Commun. 2019, 55, 3626−3629.

Co O Quantum Dots as a Highly Efficient Oxygen Evolution

17) Dhawale, D. S.; Bodhankar, P.; Sonawane, N.; Sarawade, P. B.

ACKNOWLEDGMENTS

�

4 �δ

Reaction Catalyst for Water Splitting. ACS Appl. Mater. Interfaces

017, 9, 16159−16167.

The author S. Ghosh thanks SERB-DST, India, for the

National Postdoctoral Fellowship (PDF/2017/001728/ES).

G.T., A.M., and H.R.I. acknowledge RGNF-UGC, CSIR, and

UGC for their fellowship. The authors also acknowledge IACS

for the XPS facility and IISER Kolkata for the TEM, DST-

FIST facility. S. Ghosh also thanks Heramba V. S. R. M.

Koppisetti and Ruma Ghosh for helpful suggestions.

Fast Microwave-Induced Synthesis of Solid Cobalt Hydroxide

Nanorods and their Thermal Conversion into Porous Cobalt Oxide

Nanorods for Efficient Oxygen Evolution Reaction. Sustainable Energy

Fuels 2019, 3, 1713−1719.

(18) Gong, L.; Chng, X. Y. E.; Du, Y.; Xi, S.; Yeo, B. S. Enhanced

Catalysis of the Electrochemical Oxygen Evolution Reaction by Iron

(III) Ions Adsorbed on Amorphous Cobalt Oxide. ACS Catal. 2018,

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