Journal of Applied Electrochemistry

https://doi.org/10.1007/s10800-025-02261-w

RESEARCH ARTICLE

Revolutionizing electrocatalytic synergy: optimized ­ZrO2–Co3O4

composites for high‑efficiency oxygen evolution reaction
Alvera Mahnoor1 · Fareeha Marriam1 · Khadija Munawar1 · Khurram Shahzad Munawar2,3 ·
Muhammad Adeel Asghar4 · Javed Iqbal5 · Ali Haider6 · Syed Mustansar Abbas7 · Muhammad Adil Mansoor1

Received: 3 September 2024 / Accepted: 11 January 2025

© The Author(s), under exclusive licence to Springer Nature B.V. 2025

Abstract
In the realm of innovative materials for sustainable energy applications, the synergistic integration of metal oxides has the
potential to revolutionize catalytic processes. In the pursuit of effective energy conversion, this study meticulously fabricated
and evaluated binary Z ­ rO2–Co3O4 oxides in different ratios (­ Zry:Co1−y where y = 0.1, 0.3, 0.7, 0.9) for electrocatalytic water
oxidation. Through comprehensive characterization and electrochemical studies, the excellent activity of the optimized
­Zr0.1:Co0.9 oxide catalyst was uncovered. Our findings revealed that this composite exhibited a high current density of
658 mA ­cm−2 at a potential of 1.76 V vs. RHE. This robust catalyst demonstrated an overpotential of 260 mV to achieve a
current density of 10 mA ­cm−2. Moreover high stability of catalyst was evident even after 48 h. The optimized catalysts with
low zirconium content provided better active sites for enhanced efficiency. This research highlighted the synergistic effect of
these metal oxides, offered significant potential for developing high-performance catalysts for water oxidation.
Graphical abstract

Keywords Water splitting · Composites · Zirconium oxide-cobalt oxide · Tafel slope · Electrocatalytic OER

Extended author information available on the last page of the article

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 Journal of Applied Electrochemistry

1 Introduction water splitting. These mainly include transition metal oxides

[29, 30] and hydroxides [31, 32] with exquisite efficiency
The increasing use of fossil fuels as a result of the world’s for OER and sulphides [33], phosphides [34] and carbides
expanding energy demand has led to environmental prob- [35] with high performance for HER due to their unique
lems and is a contributing factor to global warming [1–4]. electronic structure, physical properties and multi-valance
To address these challenges and support future energy needs, states. The oxides of cobalt and zirconium have been of
the focus has shifted towards renewable energy sources for great interest over the years. Cobalt oxide has demonstrated
a sustainable future [5–7]. Hydrogen has gained significant exceptional efficacy in water electrolysis because of strong
attention as a potential replacement for fossil fuels due to ­OH− binding and favorable water dissociation potential
its high energy density (142 MJ ­kg−1), compatibility with [36, 37]. But due to its natural low conductivity and lim-
electrochemical processes, and lack of ­CO2 emissions dur- ited active site exposure, it is difficult to meet the present
ing energy conversion [8–10]. However, the current industry demand particularly when it comes to current densities [38].
standard of steam reformation of hydrocarbons to produce Therefore, many efforts have been carried out to overcome
high-purity hydrogen is expensive, energy-demanding, and this obstacle. Li et al. investigated a series of different ratios
releases large amounts of ­CO2 [11]. Therefore, hydrogen of Se-doped C ­ o3O4 as OER electrocatalysts, and it was
production from electrocatalytic water splitting is a promis- found that optimized doping ratio of 6%Se–Co3O4 presents
ing and environmentally benign alternative source of energy low overpotential of 281 mV@10 mA ­cm−2 in 1 M KOH
[12–15]. solution [39]. In another work, Tongfei et al. fabricated a
Electrochemical water splitting is the most auspicious ­Co3O4-CeO2 heterostructure in situ embedded in nitrogen-
strategy involves two key reactions: the oxygen evolution doped carbon nanofibers (h-Co3O4/CeO2@N-CNFs) as a
reaction (OER) at the anode and the hydrogen evolution high-performance electrocatalyst for OER. In this study, the
reaction (HER) at the cathode [16, 17]. Commercial elec- electrochemical measurements demonstrate that h-Co3O4/
trolytic cells typically operate at voltage values of 1.8–2.0 V. CeO2@N-CNFs can perform OER activity with an overpo-
However, the theoretical voltage value for water splitting tential of 310 mV at 10 mA ­cm−2 in 0.1 M KOH solution.
at 1 atmosphere and 298 K, known as the thermodynamic [40] Similarly, Ramadoss et al. worked on porous hierarchi-
potential, is 1.23 V, which is significantly lower than the cal nanoarchitecture, constructed by ultrathin ­CoSe2 embed-
water splitting onset potential values (1.47–1.69 V vs RHE) ded Fe-CoO nanosheets (­ CoSe2@Fe–CoO). It was observed
of commercial catalysts [18, 19]. The OER at the anode that as an OER catalyst, the porous ­CoSe2@Fe–CoO hybrid
involves a thermodynamic uphill of 237 kJ ­mol−1, a multi- delivers a small Tafel of 56.2 mV ­dec−1 with an overpo-
step mechanism with several intermediates, resulting in a tential of 280 mV@10 mA ­cm−2 [41]. Furthermore, Khan
4-electron transfer process [20]. However, owing to its high et al. studied C­ o3O4/Co9S8 heterostructure for water oxida-
energy requirement and complex mechanism, the sluggish tion electrocatalysis at extremely low onset potential and it
anodic OER imposes high overpotential with slower kinetics was observed that 281 mV overpotential required to attain
in comparison to HER [21, 22]. Despite the advantages of a current density of 50 mA ­cm−2 in alkaline solution (1 M
electrochemical water splitting over steam methane reform- KOH) [42]. Moreover, various other heterostructures includ-
ing in hydrogen production, efficient catalysts are still ing CoO@Co3O4/C [43], CoO@Cu2S [44], ­TiO2@Co3O4
required to overcome the kinetic barrier and to improve the [45] and CoP@CoO [46] are observed favorable for energy
reaction kinetics, particularly for OER. application particularly OER. However, zirconium’s non-
Several considerations including cost, stability, and effi- toxicity, moisture-stable oxide, and high abundance (11th
ciency, influence the choice of electrocatalyst for water split- most abundant metal) make it exceptional for sustainable
ting [23]. Nobel metal-based nanomaterials such as Pt, Pd, technology [47]. Zirconia is emerging as strong contender
Ir, Rh and Ru are highly effective electrocatalysts for water for water splitting owing to its high stability and is able to
splitting [24, 25]. But irrespective of their exceptional activ- withstand longer in corrosive environment. Zirconia-based
ity, their high cost, scarcity, limited selectivity and short frameworks are appealing hosts for redox reactions. Z ­ r4+
durability have made widespread adoption of these catalysts exhibits a high charge-to-size ratio and strong Lewis acid
impractical [26]. Consequently, a great deal of focus has character, which could aid in stabilizing reaction intermedi-
been placed on developing more affordable and sustainable ates during the catalytic process [48]. Fe and Rh-doped Z ­ rO2
noble-metal-free catalysts. Transition metals have gained emerged as significant catalysts for OER with low overpo-
attention as alternative electrocatalysts due to their wider tential [49]. Bio-inspired NiO/ZrO2 by Zahra et al. showed
range of unpaired d-orbitals and surface-based activity in remarkable enhanced catalytic activity with 390 mV over-
electrocatalysis [27, 28]. Researchers have explored vari- potential@10 mA ­cm−2 and 72 mV ­dec−1 Tafel value. [50]
ous transition metal compounds to enhance the efficiency of Similarly, Alharbi et al. designed ­ZrO2 nano-flakes deco-
rated with L­ a2S3 to investigate OER efficacy. It has indicated
Journal of Applied Electrochemistry

a low overpotential of 280 mV@10 mA ­cm−2 with the high- contributes to improving electrical conductivity, making the
est current density of 140 mA ­cm−2 at 2.0 V vs RHE [51]. In overall composite effective for electrochemical applications.
another study, Zr-doped ­Co3O4 was also used to determine
the combined effect and it has acquired an overpotential of
307 mV to reach a current density of 20 mA ­cm−2 [52]. 2 Experimental section
It is well evident that significant efforts have been made
to develop efficient catalysts based on ­ZrO2 and ­Co3O4. 2.1 Materials and chemicals
However, there is still room to enhance the OER activity.
The previous study on Zr-doped C ­ o3O4 studied the char- Zirconium(IV) nitrate hexahydrate (Zr(NO 3) 4⋅6H 2O),
acteristics of Zr metal as dopant and did not provide any cobalt(II) nitrate hexahydrate (Co(NO3)2⋅6H2O), ethanol,
information regarding effect of varying Zr content on cata- KOH and DI water were purchased from Sigma Aldrich. For
lytic properties of ­Co3O4. The novelty of this works stems electrochemical measurements, Nafion binder solution was
into assessing the synergistic properties of Z ­ rO2 and C
­ o3O 4. purchased from Merck. The chemicals and reagents were
Despite the inherent property of zirconia, it can provide of high purity and analytical grade. All the reactants and
excellent stability to catalyst by acting as support material solvents were used as received.
and also stabilize reaction intermediates during OER [53].
Therefore it is necessary to find its optimized amount for 2.2 Preparation of composites
higher catalytic efficiency. This study helps in understand-
ing the catalytic mechanism of chemically inert and active The molar ratios of Z ­ ry:Co1−y (y = 0.1, 0.3, 0.7, 0.9) oxide
specie in a composite. For this purpose, different composites composites were prepared via the hydrothermal method. A
of ­ZrO2 and C ­ o3O4 have been synthesized via the hydrother- stoichiometric quantity of salts of zirconium and cobalt were
mal method by varying the concentration of both oxides in added to 50 mL DI water in the presence of 10 mmol urea.
the composite. Moreover, pristine oxides, ­ZrO2 and ­Co3O4 The solution was stirred until the homogenous mixture was
have also been synthesized to analyze their activity individu- obtained. The mixture was transferred to an autoclave and
ally. The current study aimed to utilize the dual efficiency placed inside a heating oven for 24 h at 180 °C. The obtained
of both oxides, Z ­ rO2 and C­ o3O4 in a composite to lower product was washed three times with DI water and one time
the activation energy barrier for effective charge transfer. with ethanol in a centrifuge machine at 8000 rpm. Then
The combination offers a synergistic effect that is beneficial the product was dried overnight in a vacuum oven and was
for catalytic applications. The formation of the composite calcined at 450 °C for 4 h with a ramping time of 30 min.
results in a greater surface area, providing more active sites Thus, pristine oxides and four composite catalysts of varying
and facilitating better adsorption of reactants. Addition- ratios were synthesized by this method and labeled as shown
ally, the chemically inherent nature of ­ZrO2 enhances the in Table 1 below.
chemical stability of the composite [52]. Meanwhile, ­Co3O4 Similarly, pure oxides of cobalt and zirconium were pre-
pared using 1.5 mmol of Co and Zr precursor separately
in 40 mL DI water then 10 mmol urea was added to it and
Table 1  Prepared catalysts along with their codes stirred until dissolved. Then the solution was transferred to
Catalysts Codes a Teflon lining autoclave and placed inside a heating oven at
180 °C for 24 h. Finally, the products were washed and dried
ZrO2–Co3O4 (Zr/Co = 0.1:0.9) Zr0.1:Co0.9
using the aforementioned procedure. The schematic diagram
ZrO2–Co3O4 (Zr/Co = 0.3:0.7) Zr0.3:Co0.7
for the synthesis of composites is shown in Fig. 1 below.
ZrO2–Co3O4 (Zr/Co = 0.7:0.3) Zr0.7:Co0.3
ZrO2–Co3O4 (Zr/Co = 0.9:0.1) Zr0.9:Co0.1

Fig. 1  Schematic diagram for the synthesis of composites

 Journal of Applied Electrochemistry

2.3 Electrode fabrication and 10,000 magnification. Energy dispersive X-ray (EDX)

was also deployed with SEM to confirm their composition.
The electrodes were prepared on a Ni-foam substrate by The electrochemical study was conducted in a three-elec-
using the drop-cast method. The Ni-foam was cut in dimen- trode electrochemical setup with platinum wire as counter,
sions of 1 × 1 ­cm2. It was washed with 1 M HCl solution Ag/AgCl as reference and modified Ni-foam as working
followed by ethanol and DI water each for 5 min under electrodes in the presence of 0.5 M KOH. Linear sweep vol-
sonication. Finally, it was kept in a vacuum oven overnight tammetry (LSV) was conducted within the potential range
for drying. The washed Ni-foam was then used to prepare of 0–0.8 V vs Ag/AgCl at 5 mV ­s−1 scan rate. Moreover,
the electrodes. To deposit the active material, 2 mg of the electrochemical impedance spectroscopy (EIS) was per-
material, 10 µL of Nafion as a binder, and 100 µL ethanol formed within the frequency range of 0.1–100 kHz under
were taken in an eppendorf tube and sonicated for 2 h until open circuit potential (OCP) with amplitude of 10 mV for
a homogenous slurry was formed. This slurry was deposited AC signal and no bias DC (DC = 0) was applied during the
on washed Ni-foam with the help of a micropipette via the measurement. Thus the tests were performed at the OPC to
drop-cast method, dried in air for 30 s and then placed in a determine the charge transfer mechanism of the catalysts.
petri dish. The petri dish was kept in a vacuum oven over-
night for drying.
3 Results and discussion
2.4 Analytical instrumentation
3.1 Microstructure and morphology
The structural properties of all synthesized catalysts were characterization
examined by X-ray diffraction (XRD) analysis with a Bragg
range of 5–80o at 1.54 Å wavelength of Cu K ­ α radiation X-ray diffraction analysis was conducted to determine the
illuminating at 20 kV voltage and 40 mA current. Raman phase purity of prepared pristine metal oxides ­(Co3O4, ­ZrO2)
spectrophotometer (RENSHAW, INVIA) with 514 nm Ar and binary metal oxide composites as shown in Fig. 2a.
laser was utilized to perform Raman spectroscopy in the Diffraction pattern of ­Co3O4 indicates resultant peaks at
range of 100–750 ­cm−1. The metal–oxygen stretching vibra- 2θ = 19.08°, 31.25°, 36.78°, 44.88°, 55.58°, 59.43°, 65.156°,
tions were identified using Fourier transform infrared spec- 68.47°, 69.9°, 76.76°, 78.43° corresponding to (111), (022),
troscopy (FTIR) through Brucker ATR-Alpha instrument in (113), (004), (224), (115), (044), (135), (244), (335), and
a frequency range of 500–4000 ­cm−1. The topographic stud- (226) planes, respectively, exhibiting the cubic structure of
ies of the catalyst were conducted by employing scanning ­Co3O4 (JCPDS: 96–900-5889). [54] The XRD pattern of
electron microscopy (SEM) of accelerated voltage 20 kV ­ZrO2 indicates the Braggs values at 24.53°, 28.11°, 31.68°,
33.94°, 41.26°, 45.14°, 50.18°, 55.38°, 60.08°, 61.70°,

Fig. 2  a X-ray diffraction pattern and b Raman analysis of C

­ o3O4, ­ZrO2, and their composites Z
­ r0.1:Co0.9, ­Zr0.3:Co0.7, ­Zr0.7:Co0.3 and Z
­ r0.9:Co0.1
in fine powder form
Journal of Applied Electrochemistry

63.16°, 70.95° ascribed to (110), (− 111), (111), (002), mixed oxidation states of cobalt ion. Survey scan was car-
(− 121), (− 202), (220), (− 013), (− 302), (− 213), (− 312), ried out in this analysis to detect all elements in the sample
and (321) planes, respectively. Peaks are matched to mono- and high-resolution spectra indicates the chemical states of
clinic structure of Z­ rO2 (JCPDS: 96–230-0297) [55]. The all elements. Survey scan as shown in Fig. 3a indicates the
XRD patterns of all binary oxides with different ratios are presence of Zr, Co and O as the primary elements in the
in good match with the standard diffraction pattern of the sample. Some of the carbon has been identified that could
pure metal oxides indicating the formation of the composite be introduced during annealing (Fig. 3b).
material. In composites, no phase shifting has been observed Figure 3c shows the high-resolution spectrum of O1s
and the intensity of planes has been varied according to the with three contributing oxygen signals. The survey has
concentration of Zr and Co in binary oxides. The phases of been dominated by O1 signal appeared at 529.2 eV. This
all binary metal oxides are in accordance with the content peak corresponds to the lattice oxygen of both ­Co3O4 and
of Zr and Co present in them. In case of Z ­ r0.1:Co0.9, greater ­ZrO2. Moreover, O2 and O3 secondary peaks have been
content of Co is confirmed by the high intensity (002), (113), observed due to surface oxygen. The peaks at 31.5 eV and
(115), and (044) phases of ­Co3O4. However, small peaks of 32 eV are associated with the hydroxyl group and absorbed
phases of Z ­ rO2 indicate that is it present in small amount oxygen, respectively, commonly observed in sample due to
in binary oxide. Similarly, as the cobalt content continued air exposure [56]. The high-resolution spectrum of Zr3d as
to decrease in other samples, the phases of ­Co3O4 become shown in Fig. 3d indicates the characteristic peaks that con-
less intense and most of them disappear in ­Zr0.9:Co0.1. In the firm the ­Zr+4 oxidation state of ­ZrO2. The doublet peak at
same way, the characteristic phases of zirconia including 181.4 and 184 eV is attributed to Z ­ r3d5/2 and ­Zr3d1/2 states,
(− 111), (111), (002), (220), and (− 302) become intense respectively [57]. Furthermore, cobalt shows the distinct
with greater Zr content in Z ­ r0.7:Co0.3 and ­Zr0.9:Co0.1. Thus, peaks of ­Co+3 and C ­ o+2 oxidation states in high-resolution
this shows that XRD also confirms the formation of binary spectra shown in Fig. 3e. The ­Co2p3/2 peak at 779.5 eV
metal oxides with desired ratios. suggests ­Co3+ as a dominant state, while the minor peak
The microstructure of the catalysts is further elucidated at 785.4 eV contributes to ­Co+2 state. Moreover, the corre-
by Raman Spectroscopy shown in Fig. 2b. The Raman bands sponding ­Co2p1/2 peaks detected at 794.5 and 796.7 eV also
located at 465 ­cm−1, 506 ­cm−1, 610 ­cm−1, and 668 ­cm−1 favored ­Co+3 and C ­ o+2 states, respectively. The presence of
+2
corresponds to the ­Eg, ­F2g(2), ­F2g(1) and ­A1g Raman active ­Co state in C ­ o3O4 has also been confirmed by presence of
modes of cubic C ­ o3O4. The peaks at 182 ­cm−1, 218 ­cm−1, shake-up satellite peaks at around 786.5 eV [58, 59].
303 ­c m , 335 ­c m −1, 380 ­c m −1, 477 ­c m −1, 537 ­c m −1,
−1
Figure 4 demonstrates the FTIR analysis of pure metal
556 ­cm−1, and 616 ­cm−1 were identified as ­Ag and ­Bg oxide and composite ­(Zr0.1:Co0.9). It is observed that Z ­ rO2
Raman active modes of monoclinic Z ­ rO2. The characteris- has exhibited absorption bands at wavelengths of 488 and
tic bands corresponding to ­Ag at 182 ­cm−1, ­Bg at 335 ­cm−1, 573 ­cm−1 regarded as stretching and bending vibration bands
­Ag at 477 ­cm−1 and A ­ g at 556 ­cm−1 depict the monoclinic of m-ZrO2. Furthermore, the band present at 678 ­cm−1 is
phase of ­ZrO2 as observed by XRD. The bands at 380 and due to the Zr–O bending vibration. The monoclinic phase of
477 ­cm−1 correspond to the stretching vibrations of m-ZrO2. ­ZrO2 is also confirmed by the characteristic stretching vibra-
The appearance of distinct peaks of C ­ o3O4 and Z ­ rO2 at the tion band of m-ZrO2 at 738 ­cm−1 [60]. The rest of the signals
same Raman shifts with varying intensity in ­Zr0.1:Co0.9, at 1338 ­cm−1, 1626 ­cm−1, and 3388 ­cm−1 are the character-
­Zr0.3:Co0.7, ­Zr0.7:Co0.3, and ­Zr0.9:Co0.1 signifies the suc- istic –OH stretching vibration of water molecules absorbed
cessful fabrication of composites. None of the composites onto the metal oxide surface. For ­Co3O4, the two significant
showed any additional oxide phase except for ­Zr0.9:Co0.1. and prominent peaks were observed. The absorption band
Unlike all other composites, this composite exhibited the at 567 ­cm−1 is due to the Co–O stretching vibration, and
formation of a cubic phase of zirconia (c-ZrO2) instead of 650 ­cm−1 shows the bridging vibration of the O–Co–O bond
a monoclinic phase as indicated by ­T2g and Eg mode of [61]. The composite’s spectrum conforms with the individ-
c-ZrO2 at 146 and 265 ­cm−1. The broad peak near 500 ­cm−1 ual independent spectra. The composite exhibited character-
is characteristic for symmetric stretching of Zr-O in cubic istic absorption bands of both ­ZrO2 and ­Co3O4 that further
­ZrO2 of This phase transition can only be attributed to the support the successful formation of the composite.
fact that cobalt oxide acts as a dopant in this case in the zir- The surface morphologies of pristine Z ­ rO2, ­Co3O4 and
conium oxide monoclinic structure, forcing it to transform their composites were scrutinized by field emission scan-
into cubic zirconia in ­Zr0.9:Co0.1 sample. ning electron microscopy as shown in Fig. 5a–l. FESEM
XPS was conducted to determine the detailed elemental micrographs of ­Co3O4 (Fig. 5a–c) show the highly compact
composition and oxidation states of the elements present in morphology of the catalyst. Figure 5d–f demonstrates that
­ZrO2–Co3O4 electrocatalyst. XPS confirms the presence of the size and shape of ­ZrO2 particles are comparatively uni-
monoclinic phase of ­ZrO2, and C ­ o3O4 with C ­ o+3 and C­ o+2 form. However, particle aggregation is evident, indicating a
 Journal of Applied Electrochemistry

Fig. 3  a XPS survey of ­ZrO2–Co3O4, High-resolution spectra of b C1s, c O1s, d Zr3d and e Co2p

potential tendency for Z­ rO2 to form clusters. This aggrega- The FESEM images of composites ­Zr0.1:Co0.9 (Fig. 5g–i)
tion may have an impact on the material’s surface area, reac- and ­Zr0.7:Co0.3 (Fig. 5j–l) indicate high surface roughness as
tivity and catalytic qualities. Moreover, the surface rough- compared to pristine materials. This roughness improves the
ness improves the electrode–electrolyte interaction which active sites for catalytic reaction, which is highly desirable
will be beneficial for greater catalytic efficiency. for OER in water splitting. Concisely, the uniform particle
size and larger surface area of these catalysts are favorable
Journal of Applied Electrochemistry

4 Electrochemical study

The electrochemical performance of the prepared cata-

lysts towards OER was investigated in a typical three-
electrode system. In this setup, the catalysts fabricated
on Ni-foam served as the working electrodes, while a
platinum wire and Ag/AgCl served as a counter and refer-
ence electrode, respectively. To determine OER catalytic
activity, linear sweep voltammetry (LSV) was carried out
within the potential range of 0–0.8 V vs Ag/AgCl at a
scan rate of 5 mV ­s−1. Figure 7a depicts the LSV curves
of ­Z rO 2, ­C o 3O 4 and their composites. The inset graph
in Fig. 7a shows the onset potential of all catalysts. It is
observed that a catalyst with 10% Zirconium and 90% Co
­(Zr0.1:Co0.9) outperformed with a high current density of
658 mA ­cm−2 at 1.74 V vs RHE. The maximum current
density with a low onset potential indicates the high OER
Fig. 4  FTIR spectra of pure Z
­ rO2, ­Co3O4, and ­Zr0.1:Co0.9 composite catalytic activity. Moreover, all composites also performed
better activity than pristine ­ZrO2 implying that less content
of cobalt inhibits the OER activity. A small hump near
for OER. Furthermore, the aggregation and homogenous 1.4 V vs RHE was observed in all catalysts. This was due
morphology point to strong structural stability, which is to the oxidation of Ni-foam which is used as substrate [62].
critical for catalyst durability. These characteristics of fab- However, an additional small hump near 1.5 V was exhib-
ricated catalysts, especially of ­Zr0.1:Co0.9 make it appropriate ited by only ­Zr0.1:Co0.9 catalyst. This could be due to sur-
for a wide range of applications. The SEM images of other face bound redox transition where surface gets activated
composites including Z ­ r0.3:Co0.7 and Z
­ r0.9:Co0.1 are given in at slightly lower potentials. This could also correspond to
supplementary file as Fig. S1. absorbed intermediate formation on the catalyst’s surface
The elemental analysis of materials has been conducted where these states stabilize before going to full oxidation
to confirm the compositional variation in all composites [63]. Furthermore, the activity of composite materials was
through EDX. The EDX spectra are shown in Fig. 6a–d. also compared with the commercial electrodes reported in
The confirmation of all Z ­ rO2–Co3O4 composites in dif- literature. The composite materials, especially ­Zr0.1:Co0.9
ferent ratios is evident from this quantitative analysis, found to be highly active in comparison to commercial Pt
exhibiting no sign of impurity. It is proved that the ele- electrode. Pt electrode reported in literature demonstrated
ments present are cobalt, zirconium, and oxygen signify- onset potential of 1.63 V vs RHE and very small current
ing the high purity. Furthermore, to briefly evaluate the density of 4 mA ­cm−2 at 1.9 V vs RHE potential [64]. The
composition of each composite, a comparison between the increased catalytic activity was attributed to the interface
theoretical and actual atomic percentages of Zr and Co is development between two oxides in the composite result-
made and provided in Table 2. Almost all samples have ing in a synergistic effect. The optimal configuration in
the desired experimental composition with little variation composites facilitates the adsorption and desorption of
concerning theoretical values. This little variations can be oxygen intermediates corresponding to the lowering of
due to the technical limitation of the EDX technique as onset potential with increased current densities. However,
it is surface sensitive technique. It can be due to sample it was noticed that the catalysts with greater zirconium
heterogeneity as the areas analyzed by EDX may have content ­(Zr0.3:Co0.7 and ­Zr0.9:Co0.1) showed less current
slightly different local composition from the bulk theo- density than pristine C ­ o3O4. The exceptional activity of
retical composition. However, the samples exhibited the ­Co3O4 in pristine or composites is due to mixed valance
successful formation of the composites of desired ratios states ­(Co2+ and C ­ o3+) which aid in the redox processes
to be investigated. required for OER [65]. ­ZrO2 itself does not have intrin-
sic catalytic properties, therefore, its higher concentra-
tion in composites is responsible for active site dilution
and diminished electrical conductivity. Therefore, Z ­ rO2
demonstrated catalytic activity at a higher potential as
compared to other catalysts. Table 3 shows the potentials
 Journal of Applied Electrochemistry

Fig. 5  FESEM images of a–c cobalt oxide and d–f zirconium oxide g–i ­Zr0.1:Co0.9 and j–l ­Zr0.7:Co0.3

required by these catalysts to reach the current densities This catalytic efficiency of ­Co3O4 becomes more notice-
of 50 and 100 mA ­cm−2 as indicated by LSV curves. The able at higher potential. Therefore, significant divergence
difference in potentials especially of ­Z rO 2 and ­C o 3O 4, is observed with ­ZrO2 exhibiting only 193 mA ­cm−2 cur-
to reach the current density of 50 mA ­cm−2 seems to be rent density at 1.75 V vs RHE while ­C o 3O 4 delivering
very small and this is attributed to the intrinsic property 384 mA/cm2 at same potential. This difference in behav-
of catalysts and their charge transfer abilities. As dis- ior at greater potentials emphasizes how intrinsic mate-
cussed earlier, ­ZrO2 is chemically inert metal oxide with rial characteristics govern catalytic performance. Simi-
restricted catalytic performance due to low to moderate larly, greater difference in terms of potential and current
electrical conductivity [66]. It does not contain the redox- density has been observed in ­Z r0.1:Co 0.9 composite due
active centers needed to facilitate the reaction efficiently. to combined effect of ­Z rO 2 and ­C o 3O 4 that resulted in
However, ­Co3O4 is active catalyst with its advantageous greater active sites and kinetics under high driving force.
redox characteristics and high electronic conductivity. However, greater zirconium content resulted in reduced
Journal of Applied Electrochemistry

Fig. 6  EDX spectra of composites in different ratios a ­Zr0.1:Co0.9, b ­Zr0.3:Co0.7, c ­Zr0.7:Co0.3 and d ­Zr0.9:Co0.1

Table 2  Comparison of theoretical and experimental atomic % ratios The optimized amount of ­ZrO2 with ­Co3O4 facilitates ion
of metals in composites transport, enhancing overall kinetics thus complementing
Composites Zr:Co ratio (Theoretical) Zr:Co ratio ­Co3O4 activity. Thus, zirconia is an effective support mate-
(Experimen- rial for other catalysts to catalyze oxygen evolution.
tal) The overpotential at different current densities was
Zr0.1:Co0.9 0.1:0.9 0.07:0.93 determined by following formula.
Zr0.3:Co0.7 0.3:0.7 0.42:0.58 Overpotential(𝜂) = ERHE − 1.23
Zr0.7:Co0.3 0.7:0.3 0.78:0.22
Zr0.9:Co0.1 0.9:0.1 0.88:0.12 Figure 7b shows a comparison of overpotential values of
all these synthesized catalysts at different current densities.
Contrary to the traditional benchmark of 10 mA ­cm−2, the
catalytically active content and reduced active sites. values of 15 and 50 mA/cm2 were chosen to find overpo-
Owing to its stable nature, it adsorbed the reaction inter- tentials because, at 10 mA ­cm−2, the LSV of all the com-
mediates strongly and excessive adsorption impedes the posites is overlapped and difficult to compare the values of
reaction kinetics and cause energy barrier. Therefore, the overpotential. The Z ­ r0.1:Co0.9 catalyst exhibited the lowest
inertness of Z
­ rO2 leads to higher overpotential require- overpotential of 302 and 353 mV to deliver the current den-
ment when present in greater amount in bimetallic oxide. sity of 15 and 50 mA ­cm−2, respectively. The overpoten-
The purpose of utilizing ­ZrO2 is to elucidate the behavior tial values for ­Zr0.1:Co0.9 are lower than that of pure ­ZrO2
of this chemically inherent substance with active specie.
 Journal of Applied Electrochemistry

Fig. 7  a LSV curves, b Overpotential comparison, c Tafel plots and d double layer capacitance of electrodes e EIS curves of ­ZrO2, ­Co3O4 and
their composites in 0.5 M KOH for OER

(η15 = 328 mV, η50 = 408 mV) and ­Co3O4 (η15 = 329 mV, the activity of pristine oxides in terms of OER. However, a
η 50 = 373 mV). Similarly, samples of different ratios greater content of zirconium leads to increased barriers to
­Zr0.3:Co0.7, ­Zr0.7:Co0.3, and ­Zr0.9:Co0.1 require overpoten- the catalytic process.
tial of 311, 318, and 333 mV to deliver 15 mA ­cm−2 cur- To have further insight into the kinetics of the OER pro-
rent density and 360, 371, and 383 mV for current density cess, the Tafel slopes of all the catalysts have been calculated
of 50 mA ­cm−2. This indicates that the composites exceed from LSV curves and shown in Fig. 7c. The smaller value of
Journal of Applied Electrochemistry

Table 3  The OER poterntial@50 and 100 mA ­cm−2 and current den- the Tafel slope suggests a smaller overpotential is needed to
sity (mA c­ m−2) at 1.75 V in 0.5 M KOH achieve greater current density, signifying greater reaction
Catalyst Potential (V) Potential (V) Current density (mA kinetics and greater efficiency. ­Zr0.1:Co0.9 has a smaller Tafel
vs RHE vs RHE ­cm−2) at 1.75 V vs slope of 102 mV ­dec−1, indicating its fastest OER kinetics
@ @ RHE among all tested catalysts. The greater Tafel slope of pristine
50 mA ­cm−2 100 mA ­cm−2
­ZrO2 (163 mV ­dec−1) and ­Co3O4 (128 mV ­dec−1) suggests
ZrO2 1.63 1.68 193 poor electron transfer, lack of active sites and hence slower
Co3O4 1.60 1.63 384 kinetics. The composite containing greater cobalt content
Zr0.1:Co0.9 1.58 1.61 599 ­(Zr0.3:Co0.7) exhibited a low Tafel slope value of 114 mV
Zr0.3:Co0.7 1.59 1.62 463 ­dec−1 in comparison to higher concentration zirconium com-
Zr0.7:Co0.3 1.60 1.63 340 posites (133 mV d­ ec−1 for ­Zr0.7:Co0.3 and 139 mV ­dec−1 for
Zr0.9:Co0.1 1.61 1.65 289 ­Zr0.9:Co0.1). High values of Tafel for ­ZrO2 and ­Zr0.9:Co0.1
indicate ineffective initial adsorption of ­OH− or production
of ­OOH− intermediates, revealing a sluggish OER process.
The optimized interaction of both oxides in composites
Table 4  Double layer capacitance and electrochemically active sur-
(as in ­Zr0.1:Co0.9) improved the availability and reactivity
face area of catalysts
of active sites. The trend of Tafel slope correlates with the
Catalyst Cdl (mF) ECSA ­(cm2) LSV findings that prove the increase in active sites in fol-
ZrO2 0.074 1.85 lowing fashion:
Co3O4 0.15 4.00
Zr0.1 :Co0.9 > Zr0.3 :Co0.7 > Co3 O4 > Zr0.7
Zr0.1:Co0.9 0.89 22.25
:Co0.3 > Zr0.9 :Co0.1 > ZrO2
Zr0.3:Co0.7 0.76 19.00
Zr0.7:Co0.3 0.13 3.32
Moreover, cyclic voltammetry was also performed
Zr0.9:Co0.1 0.08 2.00
within the non-faradic region at various scan rates to

Fig. 8  Mechanism of synergis-

­ rO2–Co3O4 for
tic effect of Z
OER (Experimental bandgap
from literature: Z­ rO2 [69],
­Co3O4 [68])
 Journal of Applied Electrochemistry

Fig. 9  a Stability curves at 1.58 and 1.61 V potential for 48 h, b Polarization curves before and after the stability test
gain insight regarding the electrochemical surface area The mechanism behind the increased activity of elec-
(ECSA) of the materials. The graphs are given in sup- trocatalyst is shown in Fig. 8. When external potential is
plementary file as Fig. S2. The plot of scan rate against applied, the electrons move from the valance band (VB)
the peak current is used to determine the slope or double to the conduction band (CB) in both the oxides leaving
layer capacitance (­ Cdl) of material as shown in Fig. 7d. behind the holes in VB. As the conduction band of ­ZrO2
Then, finally dividing the C ­ dl with specific capacitance is at higher potential than the conduction band of C ­ o 3O 4
gives the ECSA value. By sequentially following the trend, [67]. So, the electrons generated in ­ZrO2 downhill to the
­Zr0.1:Co0.9 exhibited highest ­Cdl and ECSA value as shown CB of ­C o 3O 4 thus minimizing the recombination and
in Table 4 which confirms the greater surface area of com- enhancing the charge separation. Moreover, this enhances
posite material for better catalysis. Z
­ r0.3:Co0.7 also exhib- the electron availability at C
­ o3O4 catalytic sites. The VB of
ited greater active surface area of 19 ­cm2 in comparison ­Co3O4 is significantly at higher position than VB of ­ZrO2
to pristine materials. More surface area resulted in greater thus driving the oxidation reaction [68]. So, VB of ­Co3O4
active sites and hence the overall activity of the material. is sole responsible for OER. The holes in VB of C ­ o 3O 4

Table 5  The comparison Catalyst Electrolyte Current density (mA c­ m−2) Overpoten- References
between OER η10 values and at potential (V) vs RHE tial (mV)
maximum current density of @10 mA ­cm−2
prepared electrocatalysts with
recently reported ­ZrO2 and FeOx–CoO 1 M KOH 315 at 1.7 339 [70]
­Co3O4-based materials
Co3O4–Fe2O3 1 M KOH 120 at 1.79 350 [71]
Co3O4-Ag@B1 1 M KOH 60 at 1.55 270 [72]
NixCoyO4 (x/y = 1/4) 1 M KOH 262 at 1.7 336 [73]
La-doped ­CoOx (0.2:2) 0.1 M KOH 23 at 1.6 353 [74]
La2S3–ZrO2 1 M KOH 140 at 2.0 280 [51]
CoO@Co3O4/C 1 M KOH 90 at 1.64 287 [43]
NiCo2O4/CNTs-150 0.1 M KOH 200 at 1.8 330 [75]
CoO@Cu2S 1 M KOH 180 at 1.6 277 [44]
TiO2@Co3O4 1 M KOH 90 at 1.45 270 [45]
NiO/ZrO2 1 M KOH – 390 [50]
ZrO2-ZnO-PdO 1 M KOH 120 at 1.7 370 [76]
Cu2OBi2O3ZrO2 1.5 M KOH 45 at 1.7 420 [77]
ZrO2@Co3O4 ­(Zr0.1:Co0.9) 0.5 M KOH 658 at 1.76 260 This work
ZrO2@Co3O4 ­(Zr0.3:Co0.7) 0.5 M KOH 546 at 1.76 277 This work
ZrO2@Co3O4 ­(Zr0.7:Co0.3) 0.5 M KOH 403 at 1.77 299 This work
ZrO2@Co3O4 ­(Zr0.9:Co0.1) 0.5 M KOH 346 at 1.77 316 This work
Journal of Applied Electrochemistry

remain localized and efficiently drive the oxidation reac- 5 Conclusion

tion. ­ZrO2 deep VB ensures that it acts as stable support
that prevents electron hole recombination in ­Co3O4. This study reports the successful optimization of the
Thus, the mechanism behind the increased OER activity ­ZrO2–Co3O4 composite synthesized through a hydrother-
by composite included the modified electronic structure mal method by varying the concentration of metals. The
of ­Co3O4 by adding ­ZrO2, enhancing electron density and synergistic effects in the metal oxide composites resulted
effective charge transfer. The decrease in overpotential and in improved active sites, which minimized the oxidation
Tafel slope is a clear indication of this increased charge barrier and maximized the adsorption/desorption of inter-
transfer in optimized composite material. Moreover, opti- mediates. The fabricated catalysts exhibited notable water
mizing the binding energy of oxygen intermediates by oxidation activity and reduced overpotential at high cur-
optimized amount of Z ­ rO2 can also be a significant con- rent density values. The ­Zr0.1:Co0.9 electrode achieved the
tribution to lowering the energy barrier. highest current density of 658 mA ­cm−2 at a potential of
Furthermore, the electrochemical impedance spectro- 1.76 V vs. RHE. This catalyst demonstrated an overpoten-
scopic measurements were also conducted within the fre- tial of 260 mV at a current density of 10 mA ­cm−2, show-
quency range of 0.1–100 kHz to shed some light on charge ing outstanding OER activity compared to Z ­ rO2, ­Co3O4,
transfer kinetics. Figure 7e displays the Nyquist curves of and other heterostructures. The catalysts with lower zir-
all the catalysts. There exists a relationship between charge conium content indicated significant efficiency for OER.
transfer resistance (­ Rct) and semi-circle arc diameter. Usu- These findings are expected to contribute to the expanding
ally, a small diameter signifies reduced ­Rct and better cata- body of knowledge on advanced materials for high energy
lytic efficacy. The resistance of electrolytes is represented densities.
by the high-frequency region, whilst the mass and charge
transfer events are depicted by the low-frequency region of Supplementary Information The online version contains supplemen-
tary material available at https://d​ oi.o​ rg/1​ 0.1​ 007/s​ 10800-0​ 25-0​ 2261-w.
the Nyquist plot. We can see from Fig. 6d that the trend
of charge transfer resistance for catalysts further supports Acknowledgements This work has been supported by HEC (Pakistan),
the LSV findings. The smallest semi-circle and the lowest NRPU research grant #20-12197/NRPU/RGM/R&D/HEC/2020.
charge transfer resistance is exhibited by ­Zr0.1:Co0.9 catalyst
Author contributions A. Mahnoor, F. Marriam, and M. A. Mansoor:
indicating efficient electron transfer at electrode–electro- Conceptualization, Methodology, Formal analysis, Investigation,
lyte interface. The larger semi-circles of ­ZrO2, ­Zr0.7:Co0.3, Interpretation of results, Writing—Original Draft. K. Munawar, K. S.
and ­Zr0.9:Co0.1 are a clear indication of the inefficiency of Munawar, M. A. Asghar, J. Iqbal, A. Haider, S. M. Abbas: Analysis,
the catalysts containing a high percentage of zirconium, as Resources, Electrochemical studies, Writing & Review.
they hinder charge transfer and consequently reduce OER Data availability The datasets generated during and/or analyzed during
activity. the current study are available from the corresponding author upon
The stability of ­Zr0.1:Co0.9 catalyst has also been per- reasonable request.
formed at two different potentials of 1.58 and 1.61 V for
48 h. Figure 9a shows that the catalyst exhibits excellent Declarations
stability even after 48 h. Moreover, the current density of Conflict of interest There are no conflicts to declare.
50 and 100 mA/cm2 at 1.58 and 1.61 V, respectively, cor-
related with the LSV results discussed above. This shows
that the optimized amount of ­ZrO2 with ­Co3O4 not only
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Authors and Affiliations

Alvera Mahnoor1 · Fareeha Marriam1 · Khadija Munawar1 · Khurram Shahzad Munawar2,3 ·

Muhammad Adeel Asghar4 · Javed Iqbal5 · Ali Haider6 · Syed Mustansar Abbas7 · Muhammad Adil Mansoor1

4
* Muhammad Adil Mansoor Department of Chemistry, The University of Lahore, 1‑Km,
adil.mansoor@sns.nust.edu.pk Defense Road, Lahore 54000, Pakistan
5
1 Center of Nanotechnology, King Abdulaziz University,
Department of Chemistry, School of Natural Sciences,
21589 Jeddah, Saudi Arabia
National University of Sciences and Technology (NUST),
6
H‑12, Islamabad 44000, Pakistan Department of Chemistry, Quaid-i-Azam University,
2 Islamabad 45320, Pakistan
Department of Chemistry, University of Mianwali,
7
Mianwali 42200, Pakistan Nanoscience & Technology Department, National Centre
3 for Physics, QAU‑Campus, Islamabad, Pakistan
Institute of Chemistry, University of Sargodha,
Sargodha 40100, Pakistan