# NiCo2O4 nanostructures loaded onto pencil graphite rod: An advanced composite material for oxygen evolution reaction

Zafar Hussain Ibupoto, Aneela Tahira, Aqeel Ahmed Shah, Umair Aftab, Muhammad Yameen Solangi, Jaleel Ahmed Leghari, Abdul Hanan Samoon, Adeel Liaquat Bhatti, Muhammad Ali Bhatti, Raffaello Mazzaro, et al.

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## ► To cite this version:

Zafar Hussain Ibupoto, Aneela Tahira, Aqeel Ahmed Shah, Umair Aftab, Muhammad Yameen Solangi, et al.: NiCo2O4 nanostructures loaded onto pencil graphite rod: An advanced composite material for oxygen evolution reaction. *International Journal of Hydrogen Energy*, 2022, 47 (10), pp.6650-6665. 10.1016/j.ijhydene.2021.12.024. *hal-03519731*

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Submitted on 30 Sep 2022

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NiCo₂O₄ nanostructures loaded onto pencil graphite rod: An advanced composite material
for oxygen evolution reaction

Zafar Hussain Ibupoto<sup>a*</sup>, Aneela Tahira<sup>a*</sup>, Aqeel Ahmed Shah<sup>d</sup>, Umair aftab<sup>b</sup>, Muhammad

Yameen Solangi<sup>b</sup>, Jaleel Ahmed Leghari<sup>b</sup>, Abdul Hanan Samoon<sup>b</sup>, Adeel Liaquat Bhatti<sup>c</sup>,

Muhammad Ali Bhatti<sup>e</sup>, Raffaello Mazzaro<sup>f</sup>, Vittorio Morandi<sup>f</sup>, Muhammad Ishaq Abro<sup>b</sup>, Ayman

Nafady<sup>g</sup>, Abdullah M Al-Enizi<sup>g</sup>, Mélanie Emo<sup>i</sup>, Brigitte Vigolo<sup>i*</sup>

<sup>a</sup>Dr. M.A Kazi Institute of Chemistry University of Sindh Jamshoro, 76080, Sindh Pakistan

<sup>b</sup>Mehran University of Engineering and Technology, 7680 Jamshoro, Sindh Pakistan

<sup>c</sup>Institute of Physics University of Sindh Jamshoro, 76080, Sindh Pakistan

<sup>d</sup>NED University of Engineering and Technology Karachi, Sindh Pakistan

<sup>e</sup>Department of Environmental Sciences University of Sindh Jamshoro, 76080, Sindh Pakistan

<sup>f</sup>Institute for Microelectronics and Microsystems, Italian National Research Council, Section of
Bologna, Via Piero Gobetti 101, 40129, Bologna, Italy

<sup>g</sup>Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia

<sup>i</sup>Université de Lorraine, CNRS, IJL, F-54000 Nancy, France

Corresponding authors: Zafar Hussain Ibupoto, PhD*, Brigitte Vigolo, PhD, Aneela Tahira,
PhD

Email address: zaffar.ibhupoto@usindh.edu.pk, brigitte.vigolo@univ-lorraine.fr

aneelatahira80@gmail.com

Abstract

Driving oxygen evolution reaction (OER) at extremely low overpotential and the blockage of
oxygen gas inside the catalytic material leads to the deactivation of catalytic activity, therefore it
is an essential step in electrochemical energy conversion systems, but still very challenging task.
The clay minerals including bentonite and kaolinite are rich with plenty of active centers and
favorable chemical composition for the catalysis applications but limited by the insulating
properties, thus they cannot be used as an electrode material for the water splitting. The unique
presence of clay minerals in the form of pencil graphite rod (PGR) and its attractive architecture
enabled us to exploit advantageous features and use them as an in situ electrode for growth of
metal oxide nanostructures for the electrolysis applications. The naturally inherent presence of
SiO₂ favors the catalytic properties and durability of the electrode whereas the MgO produces the
abundant oxygen vacancies and Co³⁺ ions for OER process. Herein, we present a facile approach
of using PGR as host substrate and co-catalyst for the loading of Co₃O₄, NiCo₂O₄ and NiO
nanostructures and the modified electrode carried high porosity for easily bubbling of oxygen gas,
plenty of intrinsic active centers coming from both clay minerals and metal oxides for excellent
OER process. The fabricated electrode is physically well-characterized, and it has a natural ability
to sustain a long term stability even at higher current densities and industrial electrolyzer
conditions. The NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR electrodes exhibit an overpotential of
234, 242 and 272 mV respectively at a current density of 100 mAcm⁻² in 1.0 M KOH electrolytic
solution. The presence of large number of oxygen vacancies through SiO₂ and MgO, high Ni²⁺/Ni³⁺
and Co³⁺/Co²⁺ ratios, multi metal centers, large specific surface area, high pore volume, high
electrochemical active surface area and fast charge transport within the NiCo₂O₄/PGR are the main
reasons for its superfast OER kinetics. Thus, the proposed method of electrode design will pave a
potential way for high performance electrochemical devices like metal air batteries, fuel cell and
supercapacitors.

**Keywords:** Pencil graphite rod, Metal Oxides, MgO, SiO₂, Oxygen Evolution Reaction
# 1. Introduction

Electrochemical water splitting is a route of great importance for the next generation of green energy. In the electrochemical energy conversion system, the oxygen evolution reaction (OER) is the main pillar which plays an important role in metal air batteries, photoelectrochemical cells, water splitting and fuel cell technologies [1-6]. OER is known as the limiting reaction in water electrolysis devices [7] due to the high overpotential required with most catalysts, resulting from the sluggish kinetics of the 4-electron reaction [8]. To date, precious metal-based catalysts (Ir, Ru, RuO₂ and IrO₂) are the most efficient OER catalysts [9-12] but their scalability is not feasible. For a reliable and economically sustainable water electrolysis technology, it is urgently required to develop low overpotential catalysts. Improvements in the performance of OER catalysts depend critically on the success of work aimed at finding new routes and new materials to design precious metal-free electrocatalysts. The efforts towards this goal are mainly devoted to the development of efficient earth abundant-based catalysts, as well as to the optimization of the conductive supporting material, providing both high surface area and low charge transfer resistance to the catalyst. The transition metals of first row including (Mn, Fe, Co and Ni) have received lot of attention because of their earth abundance nature and theoretically high catalytic performance [13-17]. The spinel oxides composed of transition metals like Co [18-22], Ni [23,24], Zn [25], Mn [26,27], and Fe [28, 29] have been identified as potential electrocatalysts for OER application. Co₃O₄ is one of the spinel oxides which was found to be an excellent OER electrocatalyst [30-33]. Beside this, bimetallic oxides of cobalt having a spinel structure were investigated in the wide range of energy storage and conversion applications [34-36]. Among them, nickel-cobalt bimetallic oxides of NiCo₂O₄ have been intensively studied in different electrochemical applications in the recent past [37-43]. NiCo₂O₄ is a nonprecious bimetallic oxide and it is highly active for various electrocatalysis applications such as OER, HER, ORR and alcohol oxidation reactions [44-49]. NiO was shown to exhibit a considerable catalytic activity in half-cell OER reaction [50]. However, the poor electrical conductivity and limited number of
active sites of NiO is a big barrier to capitalize it for practical applications. The Co₃O₄, NiCo₂O₄ and NiO do not match with the requirements in terms of catalytic site number, electrical conductivity, stability, and durability in order to use them for real time water electrolyzers.

The material class of clay minerals such as bentonite and kaolinite have never been combined with metal oxides for superfast water catalysis. The bentonite and kaolinite have been found to be very active adsorbents because of their excellent specific surface area, pore size and anionic surface nature [51-54]. Moreover, the chemical composition of kaolinite (Kaol), Al₂Si₂O₅(OH)₄, is associated with distinctive 1:1 layered structure formed by the stacking of Al-O octahedral and Si-O tetrahedral geometry [55]. The unique surface of kaolinite is consisting of several hydroxyl groups and they play a vital role in catalytic processes [56]. Kaolinite exhibits attractive surface features including hydrophilic surface, high surface area and considerable stability [57]. The density functional theory suggests that the presence of Al-hydroxyls in kaolinite surface shows a dynamic role in tuning the concentration of oxygen vacancies and Co²⁺ ions on Co₃O₄ surface [15].

Therefore, considering these important aspects of bentonite and kaolinite for water catalysis, we have utilized them in the shape of PGR for the deposition of the most investigated NiCo₂O₄, Co₃O₄, and NiO nanostructured materials for OER process. It is well established that the PGR are hybrid materials consisting approximately (65% graphite, 30% clay, 5% a binder (like wax, high polymer, or resins)) [58-63]. Such compounds with chemical composition, surface properties, catalytic properties, high porosity and high conductivity of PGR could be highly favorable for the OER. Also, the presence of Fe in the catalytic material can accelerate the OER activity [62].

The PGR is chemically and mechanically very stable and works at the wide range of potentials i.e. -0.8 to 0.8, -1.0 to 0.8, and -0.8 to 0.6 V versus saturate calomel electrode (SCE) in various electrolytic systems including H₂SO₄, KCl, and NaOH respectively compared to metallic electrodes such as (Au or Pt) [64]. The PGR is also a low cost electrode material among the carbon based electrodes which are utilized in the electroanalysis applications and its reported cost is 0.13 $ which is several orders less than the cost of glassy carbon electrode (GCE) 190 $ [65]. Therefore, the use of PGR as substrate and co-catalyst for the growth of metal oxide nanostructures and electrode is low cost and efficient electrode for the production of O₂. It is obvious that the bentonite and kaolinite have shown an excellent ability to host flower like morphology of NiO, thin nanosheets of Co₃O₄ and mixture of nanowires/nanowalls of NiCo₂O₄ in the current study. Both
the bentonite and kaolinite exhibit numerous catalytic centers, therefore they have been used as co-catalyst for the proposed metal oxide nanostructures.

We have found that native PGR shows the substantial OER activity relatively at high overpotential and high slope. However, the use of PGR as a substrate and co-catalyst has strongly enhanced the electrocatalytic properties of NiO, $Co_3O_4$ and $NiCo_2O_4$ electrocatalysts. Importantly, the microporosity of modified PGR has enabled the easily bubbling of $O_2$ during the measurements and enhanced the stability of electrode material for long term applications under harsh alkaline conditions.

In this work, we have developed alternative low-cost and high performance electrocatalysts. The combination of metal oxides with PGR offers particularly suitable chemical properties to surpassing OER. The proposed mechanism of the shown excellent electrochemical properties is based on the dynamic and unique chemical composition of clay minerals, and metal oxide nanostructures in the form of *in situ* composite electrode system. Moreover, an excellent electrical conductivity from graphite is favorable condition to improve the catalytic and electronic properties of $Co_3O_4$, $NiCo_2O_4$ and NiO nanostructures towards water splitting. The specific surface area obtained from standard multi-point Brunner-Emmett-Teller theory (BET) method for bare graphite pencil, NiO/PGR, $Co_3O_4$/PGR, and $NiCo_2O_4$/PGR, was found to be 19.749 $m^2 g^{-1}$, 21.711 $m^2 g^{-1}$, 22.064 $m^2 g^{-1}$ and 23.359 $m^2 g^{-1}$ respectively. The pore volume for bare graphite pencil, NiO-P, $Co_3O_4$-P, and $NiCo_2O_4$-P exhibited a 0.084 $cm^3 g^{-1}$, 1.110 $cm^3 g^{-1}$, 1.239 $cm^3 g^{-1}$, and 1.802 $cm^3 g^{-1}$ respectively was found. These experimental values about the surface area and pore volume confirm that the swift and easy transport of ions from the electrolyte through the pore size of graphite and clay minerals at the $NiCo_2O_4$ electrode surface has favored the efficient oxidation/reduction reaction by producing the intense bubbling of $O_2$ gas through the porous structure of modified PGR electrode. Scheme 1 shows the graphical representation of structure of bare and metal oxide nanostructures modified PGR, and the OER activity of composite electrodes in 1.0 M KOH.
<figure><img src="image_3.png" /><figcaption>Scheme 1 Graphical view of the prepared metal oxide/PGR composite electrodes and their use for OER process</figcaption></figure>

## 2. Results and discussion

In this study, we have grown NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR via a straightforward aqueous chemical growth method described in details in Supporting Information, section S1. The morphological and structural properties, chemical composition and functionalities of the prepared catalysts, were systematically investigated by complementary techniques such as scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) combined with energy electron loss spectroscopy (EELS) and energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), BET surface analyzer, and electrochemical measurements.
Figure 1 shows the OER overpotential of NiCo₂O₄/PGR compared to the reported outstanding OER catalysts [66--75]. The OER activity is found at the lowest overpotential of 234 mV at high current density of 100 mA cm⁻² in 1.0 M KOH aqueous solution for the NiCo₂O₄/PGR to date. Figure 1 is clearly showing the advancement in the field by introducing the new, facile, and low cost and earth abundant in situ architecture of electrocatalysts towards OER. The reported works have been found the most of the active OER catalysts, however they are produced by the complicated methods and rarely studied under the industrial electrolyzer conditions. In this view, the presented approach exhibits the lowest overpotentials and stability under the industrial electrolyzer conditions.

XRD analysis was carried out on the metal oxides deposited on the PGR (Figure 2). The bare pencil shows a typical graphitic peaks at 26.54° and 54.66° corresponding to (002) and (004) diffraction patterns, respectively. The crystal facets of MgO and SiO₂ expected from clay minerals were also identified in PGR. The reflections of MgO are completely supported by the reference card no: (96-901-3253) and the crystal facets of SiO₂ are well matched with the reference card no: (01-079-1910). The patterns of graphite are in good agreement with the reference card no: (03-065-6212). No other impurities or suspicious were determined by the XRD analysis. After the growth of onto the graphite pencil, the prominent reflections of Co₃O₄, NiCo₂O₄, and NiO were clearly identified (Figure 2). For the NiCo₂O₄/PGR, the crystal planes were identified as 111, 220, 331, 422, 511, 531, and 533. The crystal planes for the NiO/PGR were observed as 101, 012, 110, 113, and 006. The reflections of Co₃O₄/PGR were seen as 111, 022, 113, 222, 004, 224, 115, 244, 026 and 335. They are fully supported by the reference card no: [(96-900-5900), (01-073-1702), (00-044-1159)], respectively. The weak diffraction patterns of MgO, graphite carbon and SiO₂ are also recorded and they are in good agreement with the reference card no: [(96-901-3253), (01-079-1910), (03-065-6212)] as shown in Figure 2. Any other unidentified crystal facet was found in the metal oxide/PGR hybrid catalysts proving their high purity. The chemical composition of various prepared samples was quantified from XRD results using High Score Plus Software (Quantitative Rietveld analysis) to quantify the relative distribution of each component in each composite system shown in Supplementary Table 1. This analysis reveals that the amount of MgO, SiO₂ and graphite in each composite is almost constant, whereas the content of metal oxide nanostructures layer on the PGR is relatively low.
The in situ grown metal oxide nanostructures onto PGR were analyzed by SEM. The bare PGR is quite smooth with no particular nanostructures and only large platelets of clays can be observed as expected with some porous features (Supporting Information, Figure S1). While at low magnification (Supporting Information, Figure S2), no particular material structuration was observed except for PGR decorated with Co3O4 and NiCo2O4 for magnification of 10 kx (Figures S2d and S2f), high resolution (HR) SEM images reveal various nanostructures for the three designed catalysts (Figure 3). Successful synthesis of metal oxide nanostructures is of great importance for electrochemical processes thanks to the offered high surface and porosity. Interestingly, the growth at the PGR surface allowed to produce nanostructured metal oxides as observed with bulk synthesis approach [76,77]. NiO exhibit nanowalls forming flower-like structures (Figures 3a and 3b). Co3O4 are composed of thin and large nanosheets (Figures 3c and 3d) and NiCo2O4 are a mixed structure alternating large nanowalls and nanowires (Figures 3e and 3f). After the OER experiments, the observed nanostructures are basically preserved except for NiCo2O4/PGR for which the very thin nanowalls seem less stable and they probably dissolve during the electrochemical experiments (Supplementary Information, Figure S3).

The carried out EDS analysis of PGR has confirmed the successful chemical composition of raw PGR, NiO/PGR, Co3O4/PGR and NiCo2O4/PGR (Supplementary Information, Figures S4 and Supplementary Table 2). It is worth to notice that chemical composition of the nanostructured composites was found comparable even after the long term electrolysis.

XPS is an essential and powerful tool to investigate the chemical composition and the oxidation states of metal oxides. The three designed electrocatalysts were analyzed by XPS (Figure 4). The Ni 2p spectral region of NiO-PGR confirms the signature of NiO with the typical doublet structure (Figure S5a) and the Ni 2p3/2 feature showing a main line and its satellite at ~ 861 eV (Figure 4a). The main feature of Ni 2p3/2 shows the Ni2+ and the Ni3+ contribution typically centered at 853.5 eV (NiO) and 855.3 eV (Ni2O3), respectively, in good agreement with the literature [78-80]. From deconvolution, the Ni2+ / Ni3+ ratio was estimated to be 0.36. As expected, the O 1s profile of NiO/PGR exhibits two peaks at 529.3 and 531.0 eV (Figure 4b). They are respectively assigned to the metal-oxygen bonds and singly charged oxygen vacancies [81, 15]. The amount of oxygen vacancies in the NiO/PGR was found to be of 33%.

For Co3O4/PGR, Co 2p and O 1s spectra are shown in Figures 4c and 4d (the 2p spectral region Co3O4/PGR is shown in Figure S4b). Co 2p3/2 doublet was well fitted by the tetrahedral Co3+ (779.4
eV) and octahedral Co²⁺ (780.4 eV) contributions [80-82]. Co³⁺/Co²⁺ ratio is found to be ~ 0.27. The O 1s peak was de-convoluted into two peaks positioned at 529.8 and 530.9 eV which corresponds to the Co-O bonds and oxygen vacancies, respectively [83]. The Co₃O₄/PGR electrode exhibits 49 % of oxygen vacancies.

De-convolution of Co 2p<sub>3/2</sub>, Ni 2p<sub>3/2</sub> and O 1s signals of NiCo<sub>2</sub>O<sub>4</sub>/PGR shows the main spectral signatures expected for this NiCo<sub>2</sub>O<sub>4</sub> (Figures 4e, 4f and 4g, the full spectral domain of Co 2p and Ni 2p is shown in Figures S5c and S5d) [84-86]. The Ni 2p<sub>3/2</sub> feature was fitted by four components corresponding to the Ni<sup>2+</sup> ions at 853.8 eV, the Ni<sup>3+</sup> ions at 855.4 eV and the shake-up satellite peak deconvoluted with two lines (861.0 and 866.2 eV). The main lines from the Co 2p<sub>3/2</sub> fits are as well consistent with Co<sup>3+</sup> (779.2 eV) and Co<sup>2+</sup> (280.2 eV) species usually found for NiCo<sub>2</sub>O<sub>4</sub> oxide. From the respective area feature, Ni<sup>2+</sup>/Ni<sup>3+</sup> and Co<sup>3+</sup>/Co<sup>2+</sup> ratios have been found around 0.19 and 0.37, respectively. The high Ni<sup>2+</sup>/Ni<sup>3+</sup> and Co<sup>3+</sup>/Co<sup>2+</sup> ratios in the NiCo<sub>2</sub>O<sub>4</sub>/PGR has revealed dynamic contribution towards enhanced OER activity. The O 1s spectra recorded from NiCo<sub>2</sub>O<sub>4</sub>/PGR materials could be well fitted with two oxygen contributions corresponding to binding energies of 529.4 and 530.9 eV corresponding to the O-Ni/Co bonds and the oxygen vacancies, respectively. The quantified 52 % of oxygen vacancies was observed in the NiCo<sub>2</sub>O<sub>4</sub>/PGR sample.

We report here detailed characterizations at the atomic scale of NiCo₂O₄/PGR before and after
OER experiments by HRTEM and scanning transmission electron microscopy (STEM) combined
with EELS and EDS. HRTEM micrographs reveal that NiCo₂O₄ clusters are highly crystallized
either before or after OER experiments (Figures 5a-5b,5g-5h). The spacing distances d_hkl obtained
from the Fast Fourier Transform (FFT) patterns (insert Figures 3b and 3h) correspond to the cubic
structure of NiCo₂O₄ in agreement with XRD experiments (Fdm, a = b = c = 8.1 Å). Additionally,
EELS maps show that Ni, Co and O are homogeneously distributed within the particles for both
samples (Figures 5d-5f and 5j-5l). The TEM images at different magnifications are enclosed in the
Supplementary Information, Figure S6. They provide the local morphological features of
NiCo₂O₄/PGR before the OER analysis. These images show that the NiCo₂O₄/PGR is mainly
composed of nanoparticles with size of 10 nm which certainly facilitate the easy access of catalyst
surface for the water splitting reaction. Furthermore, the shape of nanowalls and needles at higher
scale is highly favorable for the transport of charges during the water catalysis. Supplementary
EDS experiments were done over larger areas of the samples and they are in complete agreement
with EELS results (Figures S7a-S7f). These in-depth structural and elemental analysis
demonstrates the high stability of the designed electrocatalyst.

The nanostructured electrocatalyst performance is extremely dependent on the porosity and
particle/pore size of the electrocatalyst. The specific surface area and the pore-size distribution of
the bare graphite pencil, NiO/PGR, Co3O4/PGR, and NiCo2O4/PGR nanostructures were
calculated by nitrogen gas adsorption experiments; as shown in Figure 6. A type-II isotherm was
obtained for all nanostructures, the presence of macropores the isotherm rises rapidly near P/P₀ =
1 and in the limit of large macropores may exhibit an essentially vertical rise. Therefore, it is
anticipated that the NiO/PGR, Co3O4/PGR, and NiCo2O4/PGR have larger macroporous
structures. Furthermore, NiO/PGR, Co3O4/PGR, and NiCo2O4/PGR isotherms reveal Type H3
hysteresis which is due to the non-rigid aggregates of plate-like particles and slit-shaped pores.
These results are in agreement with the SEM results. The specific surface area, calculated using
the standard multi-point BET method for bare graphite pencil, NiO/PGR, Co3O4/PGR, and
NiCo2O4/PGR, was found to be 19.749 m² g⁻¹, 21.711 m² g⁻¹, 22.064 m² g⁻¹ and 23.359 m² g⁻¹
respectively. Bare graphite pencil, NiO-P, Co3O4/PGR, and NiCo2O4/PGR exhibited a 0.084 cm³
g⁻¹, 1.110 cm³ g⁻¹, 1.239 cm³ g⁻¹, and 1.802 cm³ g⁻¹, pore volume, respectively (inset of Figure
6). This suggested that the NiCo2O4/PGR composite accelerated the OER activity due to its high
specific surface area [87-90].

**2.1.Half-cell OER characterization**

The exposure of hydroxyl groups from highly porous clay structure enabled favorable interaction
with metallic ions during the growth process and consequently a compatible metal oxide-PGR
composite is prepared. The prepared composite electrodes have several unique features for electro
catalytic applications such as high specific surface area, multiple number of active sites, excellent
electrical conductivity, high metal oxide-PGR compatibility, abundant oxygen vacancies, higher
oxidation states of transition metallic ions and large porosity. The electrocatalytic activities of bare
graphite pencil and NiCo2O4/PGR, Co3O4/PGR, and NiO/PGR were evaluated in 1.0 M KOH
aqueous solution as shown in Figure 7. The *in situ* grown Co3O4 and NiCo2O4 on graphite pencil
have shown an excellent OER activity with a large current density and the lowest overpotential
than the other catalysts as shown in Figure 1. The NiCo2O4/PGR, Co3O4/PGR, and NiO/PGR
produce a current density of 100 mA cm⁻² at an overpotential of 234 mV, 242 mV, and 272 mV,
respectively. The PGR exhibits a high porosity which enables the ionic charges flow at the applied potential which might create a current prior to the OER onset potential as shown in Figure 7. The bare PGR needs an overpotential of 441 mV to produce a current density of 100 mA cm⁻². The overpotential analysis suggests that the NiCo₂O₄/PGR, and Co₃O₄/PGR have 38 mV, 30 mV lower overpotential at a current density of 100 mA cm⁻² in 1.0 M KOH than NiO/PGR catalyst. The high Ni²⁺/Ni³⁺ and Co³⁺/Co²⁺ ratios and 52 % of oxygen vacancies in the composition of NiCo₂O₄/PGR have favored improved OER performance. It has been proved that the high content of Co³⁺ in the cobalt based catalysts can enhance the OER because Co³⁺ ions provide a high probability for the adsorption of electrophilic species and consequently they facilitate of oxygen in the OH⁻ ions ⁷³, ⁷⁴. Furthermore, the main challenge of blockage of oxygen gas inside the catalyst surface is resolved by the proposed PGR based material due to its large pore volume which easily bubble out the oxygen gas without the deactivation of catalytic material as clearly described by BET results. Therefore, the modified PGR electrode can be used for the long term applications.

The NiCo₂O₄/PGR electrode is associated with large specific surface area and pore volume of 23.359 m² g⁻¹ and 1.802 cm³ g⁻¹ respectively and they have played a vital in accelerating the OER kinetics and stability. The high specific surface of area of NiCo₂O₄/PGR has provided the large exposure of active centers for the catalytic reaction, therefore an excellent OER activity is demonstrated [91, 61]. Furthermore, the modification of PGR with nanostructured materials is associated with the high specific surface area, electrocatalytic properties and good conductivity compared to the bare PGR, thus an enhanced electrochemical activity is observed [92]. The XRD study has revealed the presence of SiO₂ and MgO in the bare and modified PGR electrodes which have been shown to significantly impact the electrocatalytic performance. The reactive distribution of SiO₂, MgO and graphite from the XRD results in each composite system was found nearly same, indicating that most of the catalytic activity towards OER was coming from layer of nanostructured metal oxides. But, at the same time we have seen in the literature that the SiO₂ and MgO have played role in lifting the catalytic activity, therefore in our composite systems their role was identified as co-catalyst for various metal oxide nanostructures. The silica material has tendency to allow the water molecules due to its high wettability which further gives out an active oxide layer with high porosity structure [93]. This high porosity of active oxide coating features within the bulk depth of PGR electrode and lead to the enhanced electrocatalytic performance. The high surface energy of silica from the PGR and interlocked structure between the NiO, Co₃O₄, NiCo₂O₄
and inactive silica reveal the excellent adhesion [94]. The presence of silica in the electrode not only improve the catalytic properties but it also increases the durability towards long term applications [94]. Beside this, a certain amount of MgO in the electrode is essential to produce a high amount of oxygen vacancies via induced defects engineering [95] and abundant concentration of Co³⁺ ions. The presence of MgO within the composition of composite systems could provide defects and edges, thereby it enhanced the OER activity of metal oxide nanostructures [95]. The oxygen vacancies have been shown to decrease the adsorption energy of water molecules and become responsible for the weakening of metal-oxygen bonds to provide the favorable environment for the exchange of short lived species and electrons, therefore enhancing the OER reaction [96, 97]. Furthermore, the presence of iron from bentonite in the composite systems favored the superior OER activity [62].

This outstanding performance of NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR is attributed to the unique features of fabricated electrode consisting clay minerals, graphite, and metal oxide nanostructures. For the better pictorial presentation of obtained overpotential values at 100 mA cm⁻² and they are plotted against the electrode materials such as bare pencil, NiCo₂O₄, Co₃O₄, and NiO composite materials as shown in Figure 7b. Further to add, the nanowires like morphology in case of NiCo₂O₄ onto pencil graphite is guiding the electrochemical reaction at favorable kinetics, thus low overpotential was observed. The OER kinetics on the surface of NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR systems was evaluated by Tafel plots under alkaline conditions. The NiCo₂O₄ composite material exhibits a Tafel slope of 110 mVdec⁻¹, Co₃O₄ composite possesses a Tafel slope of 132 mVdec⁻¹ and NiO composite has shown Tafel slope of 115 mVdec⁻¹ as shown in Figure 7c. The obtained Tafel slopes are slightly higher due to several reasons such as microstructure of electrode and the composition of electrode. The OER mechanism is not well understood in the literature, however generally it involves the 4-electron transfer process and followed by the high energy barrier of overpotential [98,99]. In the alkaline electrolytic conditions, the OER takes place through the oxidation of adsorbed hydroxyl anion on the metal active center with the formation of M-OH specie as shown in equation (a). Afterwards, the proton and electron are taken away from the M-OH to give out M-O as shown in equation (b). The formation of molecular oxygen is followed by either of the two pathways. In one pathway, M-O interacts with a hydroxyl anion to produce hydroperoxide M-OOH, then proton- electron couple transfer step takes place with the release of molecular oxygen as shown in equation (c) and (d) [100, 101]. In
the second pathway, the direct association of two M-O intermediates gives out the molecular oxygen as shown in equation (e) [102].

$$ \mathrm{OH}^{-} \rightarrow \mathrm{M}-\mathrm{OH}+\mathrm{e}^{-} \quad (\mathrm{a}) $$

$$ \mathrm{M}-\mathrm{OH}+\mathrm{OH}^{-} \rightarrow \mathrm{M}-\mathrm{O}+\mathrm{H}_{2} \mathrm{O}+\mathrm{e}^{-} \quad (\mathrm{b}) $$

$$ \mathrm{M}-\mathrm{O}+\mathrm{OH}^{-} \rightarrow \mathrm{M}-\mathrm{OOH}+\mathrm{e}^{-} \quad (\mathrm{c}) $$

$$ \mathrm{M}-\mathrm{OOH}+\mathrm{OH}^{-} \rightarrow \mathrm{O}_{2}+\mathrm{H}_{2} \mathrm{O}+\mathrm{e}^{-} \quad (\mathrm{d}) $$

$$ \mathrm{M}-\mathrm{O}+\mathrm{M}-\mathrm{O} \rightarrow \mathrm{O}_{2} \quad (\mathrm{e}) $$

The Tafel slopes for these composite materials are estimated from the region of large current densities, therefore the measured value of Tafel slope is slightly higher. Another reason for the high value of Tafel slope is inherently assigned to the insulation offered by the clay minerals in the modified electrode. This analysis again opens, new domains for the future studies on the optimization of clay composition in the PGR in order to bring Tafel value at low magnitude which will further facilitate the understanding of OER mechanism in alkaline electrolytic conditions. To understand the reason for the significant OER activity of newly prepared composite materials NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR, the electrochemically active surface area (ECSA) was calculated as it can be a good indicator for the evaluation of significant changes at the interface of electrode [80,81]. The ECSA was calculated from the non-Faradic region of CV curves at various scan rates in 1.0 M KOH (Supplementary Information, Figure S8). The material with higher value of ECSA exhibits a superior electrocatalytic activity because of the large surface area and high exposure of catalytic sites during electrocatalysis. The NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR possess an ECSA of 736.1 µFcm⁻², 522.2 µFcm⁻², and 223 µFcm⁻² respectively as shown in Figure 7d. The bare graphite pencil exhibits an ECSA value of 142.9 µFcm⁻². It is obvious from ECSA values that the composite materials have an excellent electrocatalytic activities due to higher surface area compared to the pristine metal oxides. Figure 7e shows a multistep
chronopotentiometric response of NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR with a starting
current density of 100 mA cm⁻² to 450 mA cm⁻² (100 and 50 mA cm⁻² per 500 s) in 1.0 M KOH.
The potentials became immediately leveled off at 1.46 V, 1.47 V and 1.50 V for NiCo₂O₄/PGR,
Co₃O₄/PGR, and NiO/PGR respectively at the beginning of current value and remained unchanged
for 500 s and other steps also demonstrated the same behavior indicating an excellent electrical
conductivity, mechanical activity and mass transfer of NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR.
It is worth to see that the rough line at higher current densities that of 100 mA cm⁻² is suggesting
the swift bubbling of gas during the OER processes. But it is not much intense which indicates
that the produced oxygen is successfully removed from the surface due to the porous nature of
electrode where oxygen bubble produced inside the porous catalyst structure is evacuated with
negligible limitation factor offered by porous throat. The multistep potential curves were recorded
for the NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR in 1.0 M KOH as shown in Figure 7f and they
have fully supported the results shown in Figure 7e. The enhanced electrocatalytic activity of
NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR was further elucidated by electrochemical impedance
spectroscopy (EIS) in 1.0 M KOH aqueous solution. Figure 7g represents the Nyquist plots of
NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR, thereby the charge transfer resistance (Rct) of prepared
materials between the interface of electrode and electrolyte was quantified. The Rct of
NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR in OER was found in the order of 4.87, 6.15, and
12.25Ω cm² respectively. However, a large charge transfer resistance of bare pencil 306.9 Ω cm²
in OER was observed. The charge transfer of bare pencil graphite rod is higher than three
composite samples due to fact that the nanostructured materials have better electron
communication due to quantum confinement effect. Further to add, the nanowires like
morphology in case of NiCo₂O₄ onto pencil graphite is guiding the electrochemical reaction at
favorable kinetics. The quantum confinement effect is well established in the literature when the
electrical communization features are described. These results verify the feasibility of swift
electron transport in the composite samples compare to the bare graphite pencil due to the mutual
contribution of electrical conductivity from semiconducting metal oxides and graphite. In concise,
the enhanced catalytic activity of composite materials in OER can be associated to the different
combined effects of increased electrical, surface and chemical properties in the composite
materials that successfully capitalize the OER features at the same time. Before and after the
durability test, the LSV curves were found stable for the composite materials NiO, Co₃O₄, and
NiCo₂O₄ without any loss of current density and onset potential as shown in Figure 8a,b,c. The long term durability of composite materials NiO, Co₃O₄, and NiCo₂O₄, was also studied at constant current density of 100 mA cm⁻² in 1.0 M KOH and no any abrupt change in the overpotential was recorded for the time period of 60 h as shown in Figure 8d,e,f. These stability results suggest that the NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR electrodes are capable to work for long term applications and the structural and compositional studies were also carried out after the durability measurements. All the modified electrodes have maintained the morphology and composition after durability test as shown in Figure S3 and S4 respectively. The NiCo₂O₄/PGR electrocatalyst before and after the OER experiment was further studied in term of morphology and composition by the HRTEM and EELS. The obtained results revealed the high durability and stability of material without the loss of morphology and composition during long term OER testing as shown in Figure S7. The stability and durability of composite electrodes could be attributed to high metal oxide-PGR compatibility and easy bubbling of O₂ gas through the porous structure which did not change or damage the composition and morphology of composite electrodes.

The concentration of electrolyte has significant effect on the current density, overpotential and stability of electrocatalyst towards any electrolysis process. It has been known that highly alkaline electrolyte strongly decrease the activity and stability of nonprecious metal oxides based electrocatalysts for OER reaction. For this reason, we have investigated the role of KOH electrolytic concentration on the performance of presented metal oxides decorated on the PGR. The two concentrations of electrolyte were selected such as 0.1 M KOH and 6.0 M KOH. The purpose of using 0.1 M KOH was to see effect on the catalytic activity of the NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR electrodes, however the use of 6.0 M KOH electrolytic concentration was to monitor the performance of each catalyst under industrial electrolyzer conditions. This study could help us to identify the most efficient material for the industrial oxygen production. The LSV curves were measured for NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR at a scan rate of 5 mV/s in 0.1 M KOH see Supplementary Information, Figure S9). The calculated overpotentials at 30 mA cm⁻² for the NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR were 290 mV, 310 mV and 330 mV respectively. This suggests that the electrolyte concentration has significant influence on the current density, overpotential and stability of electrocatalyst, therefore it will provide a guideline that the proposed materials could be tested at the higher alkaline electrolyte concentrations.
Therefore, we have also investigated the catalytic activities of NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR under 6.0 M KOH electrolytic conditions as shown in Figure 8a. These newly prepared NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR have shown the lowest overpotentials of 210 mV, 220 mV, and 250 mV respectively for a current density of 150 mA cm⁻² as shown in Figure 9a. The concentration of an electrolyte has significantly decreased the overpotential and increased the current density of NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR composite materials in 6.0 M KOH solution. This indicates an outstanding OER activities of newly proposed electrodes and provide an easy access of these efficient materials for industrial applications. The NiCo₂O₄/PGR exhibits a superior performance than the other composite electrode materials due to favorable electronic environment of bi-metal ions and larger surface area. The NiCo₂O₄/PGR has lower effective current density value due to its large size, thus we see a low overpotential for it.

The temperature has significant effect on the activity of electrocatalysts. Moreover, the effect of temperature on the electrocatalytic activity of these composite catalysts was also studied. It was also found that the OER activity of these composite materials in 6.0 M KOH was highly enhanced at a high temperature of 60 °C and the NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR required a lower overpotential of 160 mV, 180 mV, and 210 mV, respectively to achieve a current density of 250 mA cm⁻² as shown in Figures 9b-9d. The steady-state performance and durability of NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR was also evaluated by multiple current steps chronopotentiometry experiments in 6.0 M KOH. It can be seen from Figure 9e, the variation in the increment of applied potential was recorded when the electrocatalytic current density was lifted to higher value ranging from 150 to 900 mA cm⁻². At the initial step, the 150 mA cm⁻² was leveled off by the overpotential of 1.44 V, 1.45 V, and 1.48 V for the NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR respectively and remains static for the time interval of 500 s. The increasing multiple current steps guided to keep overpotentials constant at a higher value than that of 150 mA cm⁻². Also, the multistep potential experiment was conducted in order to verify the results shown in Figure 9f and the obtained results successfully demonstrated close in agreement for achieving the desired overpotential at specific current density. These catalytic activities reveal the outstanding performance, mass transport features and intrinsic excellency of NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR during the long term OER electrolysis.

### 3. Conclusions
We have used pencil graphite rod with favorable properties for OER which are summarized as: 1. Iron active centers from bentonite played a drastic role in enhancing OER activity of NiCo₂O₄, Co₃O₄, and NiO, 2. The bentonite and kaolinite are associated with attractive surface properties, thermal and structural stability, which effectively hosted the NiCo₂O₄, Co₃O₄, and NiO nanostructures, 3. The graphite has driven the electrical conductivity of PGR modified with NiCo₂O₄, Co₃O₄, and NiO. The measured low overpotential of 234 mV at high current density of 100 mA cm⁻² for NiCo₂O₄/PGR in 1.0 M KOH aqueous solution demonstrates a significant OER activity for a nonprecious material. The large pore volume of NiCo₂O₄/PGR prevents the blockage of oxygen gas due to the porosity of modified electrode which easily bubbled out produced oxygen gas without the loss of activity of composite catalyst and its stability during OER process, therefore it can be used for long term applications. The superior performance of these composites towards OER is due to high surface to volume ratio due to nanostructured metal oxides and microporosity of PGR, high density of active sites contributed from clay minerals of PGR and metal oxides. The multi-step durability using both chronoamperometry and chronopotentiometry revealed the unseen role of bentonite and kaolinite in the enhancement of OER activities. Moreover, the optimization of bentonite, kaolinite and graphite ratio in the composition of PGR can even lower the overpotential of OER compared to the obtained values. The naturally inherent presence of SiO₂ favors the catalytic properties and durability of the electrode whereas the MgO produces the abundant oxygen vacancies and Co³⁺ ions. The presence of large number of oxygen vacancies through SiO₂ and MgO, high Ni²⁺/Ni³⁺ and Co³⁺/Co²⁺ ratios, multi metal centers, large specific surface area, high pore volume, high electrochemical active surface area and fast charge transport within the NiCo₂O₄/PGR became the main reasons for its superfast OER kinetics. The proposed strategy of fabrication of low cost, earth abundant, efficient and stable catalysts for OER and other electrochemical applications can be considered as a promising and alternative guideline.

## Acknowledgments

We would like to thank the platform "Spectroscopies et Microscopies des Interfaces" (Laboratory of Physical Chemistry and Microbiology for Materials and the Environment, LCPME, Nancy, France) and A. Renard, Dr. M. Mallet (LCPME) for XPS analysis. B.V. thanks the platform "Microscopies, Microprobes and Metallography (3M)" (Institut Jean Lamour, IJL, Nancy, France)
and Dr. J. Ghanbaja for fruitful discussions. We also wish to thank Researchers Supporting Project number (RSP-2021/55) at King Saud University, Riyadh, Saudi Arabia.

**Declaration Statement**

Authors declare that the presented work is original and only considered for this journal.

**Data Availability Statement**

The processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

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## Figure Captions

**Figure 1** The comparison of Co₃O₄ and NiCo₂O₄-P in terms of overpotential with already well-established electrocatalysts for OER at 100 mA cm⁻² in 1M KOH aqueous solution (P stands for PGR).

**Figure 2** Powder XRD patterns of bare PGR, NiCo₂O₄-P, Co₃O₄-P, and NiO-P (P stands for PGR).
**Figure 3** SEM images taken at different magnifications of the catalysts after their growth on PGR: a) and b) NiO-P, c) and d) Co₃O₄-P and e) and f) NiCo₂O₄-P (P stands for PGR).

**Figure 4** XPS spectra (a) Ni 2p₃/₂ and (b) O 1s of NiO-P; (c) Co 2p₃/₂ and (d) O 1s of Co₃O₄-P; (e) Ni 2p₃/₂, (f) Co 2p₃/₂ and (g) O 1s of NiCo₂O₄-P (P stands for PGR).

**Figure 5** HRTEM micrographs (a and g), zoom-in of the green square area (b and h) (insert: corresponding Fast Fourier Transform (FFT)), TEM micrographs at low magnification (c and i) and corresponding EELS elemental maps of cobalt (d and j), nickel (e and k) and oxygen (f and l) of NiCo₂O₄-P before (a-f) and after the OER reactions (g-l) (P stands for PGR).

**Figure 6** BET analysis of a. bare graphite pencil, b. NiO-P, c. Co₃O₄-P, and d. NiCo₂O₄-P (inset shows the pore size distribution) (P stands for PGR).

**Figure 7** a. LSV curves of bare graphite pencil, NiO-P, Co₃O₄-P, NiCo₂O₄-P, 20%wt RuO₂/C at a scan rate of 5mVs⁻¹ in 1.0M KOH at 25 °C, b. Plot of overpotential @ 100 mAcm⁻² versus bare graphite pencil, NiO-P, Co₃O₄-P, NiCo₂O₄-P, c. Tafel plots in 1.0M KOH, d. Linear plot of current density versus different scan rates, e. multi-step chronoamperometry measurement in 1.0M KOH beginning from @ 100 mAcm⁻² for the time interval of 500 s and with consecutive rise of @50 mAcm⁻², f. Multistep potential chronopotentiometric curve of NiO-P, Co₃O₄-P, NiCo₂O₄-P in 1.0M KOH at the beginning of 1.48 V vs RHE during each step and time interval of 500 s, g. Nyquist plots of bare graphite pencil, NiO-P, Co₃O₄-P, NiCo₂O₄-P in 1.0M KOH, inset showing the fitted equivalent circuit model (P stands for PGR).

**Figure 8** a. LSV curves of NiO-P before and after the durability test at a scan rate of 5mVs⁻¹ in 1.0 M KOH at 25 °C, b. LSV curves of Co₃O₄-P before and after the durability test at a scan rate of 5mVs⁻¹ in 1.0 M KOH, c. a. LSV curves of NiCo₂O₄-P before and after the durability test at a scan rate of 5mVs⁻¹ in 1.0 M KOH, d. chronoamperometric response of NiO-P in 1.0M KOH @ 100 mAcm⁻², e. chronoamperometric response of Co₃O₄-P in 1.0M KOH @ 100 mAcm⁻², f. chronoamperometric response of NiCo₂O₄-P in 1.0M KOH @ 100 mAcm⁻² (P stands for PGR).
**Figure 9** a. LSV curves of bare graphite pencil, NiO-P, Co₃O₄-P, NiCo₂O₄-P, at a scan rate of 5mVs⁻¹ in 6.0M KOH at 25 °C, b. LSV curves of Co₃O₄-P at a scan rate of 5mVs⁻¹ in 6.0M KOH at 25 °C and 60 °C, c. LSV curves NiCo₂O₄-P at a scan rate of 5mVs⁻¹ in 6.0M KOH at 25 °C and 60 °C, d. LSV curves of NiO-P at a scan rate of 5mVs⁻¹ in 6.0M KOH at 25 °C and 60 °C, e. Multistep current chronoamperometric response of NiO-P, Co₃O₄-P, NiCo₂O₄-P in 6.0M KOH at the beginning of 150 mAcm⁻² during each step and time interval of 500 s, f. Multistep potential chronopotentiometric curve of NiO-P, Co₃O₄-P, NiCo₂O₄-P in 6.0M KOH at the beginning of 1.48 V vs RHE during each step and time interval of 500 s (P stands for PGR).
<figure><img src="image_4.png" /><figcaption>Figure 1</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 3</figcaption></figure>
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>
<figure><img src="image_12.png" /><figcaption>Figure 4</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Figure 5</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Figure 6</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Figure 7</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Figure 8</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>Figure 9</figcaption></figure>
**Supplementary Information**

**NiCo₂O₄ nanostructures loaded onto pencil graphite rod: An advanced composite material for oxygen evolution reaction**

Zafar Hussain Ibupotoᵃ*, Aneela Tahiraª*, Aqeel Ahmed Shahᵈ, Umair aftabᵇ, Muhammad Yameen Solangiᵇ, Jaleel Ahmed Leghariᵇ, Abdul Hanan Samoonᵇ, Adeel Liaquat Bhattiᶜ, Muhammad Ali Bhattiᵉ, Raffaello Mazzarof, Vittorio Morandif, Muhammad Ishaq Abroᵇ, Ayman Nafadyg, Abdullah M Al-Enizig, Mélanie Emoi, Brigitte Vigoloi*

ªDr. M.A Kazi Institute of Chemistry University of Sindh Jamshoro, 76080, Sindh Pakistan

ᵇMehran University of Engineering and Technology, 7680 Jamshoro, Sindh Pakistan

ᶜInstitute of Physics University of Sindh Jamshoro, 76080, Sindh Pakistan

ᵈNED University of Engineering and Technology Karachi, Sindh Pakistan

ᵉDepartment of Environmental Sciences University of Sindh Jamshoro, 76080, Sindh Pakistan

fInstitute for Microelectronics and Microsystems, Italian National Research Council, Section of Bologna, Via Piero Gobetti 101, 40129, Bologna, Italy

gDepartment of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia

iUniversité de Lorraine, CNRS, IJL, F-54000 Nancy, France

Corresponding authors: Zafar Hussain Ibupoto, PhD*, Brigitte Vigolo, PhD, Aneela Tahira, PhD

Email address: zaffar.ibhupoto@usindh.edu.pk, brigitte.vigolo@univ-lorraine.fr
[aneelatahira80@gmail.com](mailto:aneelatahira80@gmail.com)

**S1. Materials and methods**

**Materials.** The used chemicals include cobalt chloride hexahydrate, nickel chloride hexahydrate, potassium hydroxide, alumina slurry (0.05µm), ethanol 99.99%, and 5% Nafion membrane were
purchased from Sigma Aldrich Karachi, Pakistan. These chemicals were of analytical grade and
used without any further purification. The Castell 9000 graphite pencil, 7B was made in Germany
and purchased from a local market of Hyderabad, Sindh, Pakistan. The pencil graphite leads are
composite materials consisting approximately (65% graphite, 30% clay, 5% a binder (like wax,
high polymer, or resins). The desired solutions were prepared in the deionized water. The chemical
composition of kaolinite (Kaol) is $Al_2Si_2O_5(OH)_4$ with distinctive 1:1 layered structure formed by
the stacking of Al-O octahedral and Si-O tetrahedral geometry. The unique surface of kaolinite is
consisting several hydroxyl groups, hydrophilic surface, high surface area, good stability and they
play a vital role in catalytic processes. On the other hand, bentonite is aluminum phyllosilicate clay
and it mostly contains montmorillonite. The bentonite is inherited by Fe impurity in its chemical
composition; therefore, it can play a vital role during water splitting.

**Synthesis of metal oxide nanostructures onto graphite pencil.** The wooden part of Castell 9000 graphite pencil, 7B was removed and naked graphite rod was cleaned with ethanol and washed several times with the deionized water. Then, graphite rod was dried at room temperature. For the growth of $Co_3O_4$, the precursor solution of 0.1 M cobalt chloride hexahydrate and 0.1 M urea was prepared in 100 mL of the deionized water. For the growth of NiO, the 0.1 M nickel chloride hexahydrate and 0.1 M urea was added to 100 mL of the deionized water. For the growth of $NiCo_2O_4$, the precursor solution of 0.1 M cobalt chloride hexahydrate and 0.1 M urea was prepared in 100 mL of the deionized water. Then 0.015 M nickel chloride hexahydrate was also added in this solution. Afterwards, cleaning the pencil graphite rods, they were vertically dipped in each precursor beaker and covered with the aluminum foil. Then growth solutions were kept at 95 °C for 5 h. After the completion of growth time, the graphite rods were coated with dense metal hydroxide nanostructures and washed several times with the deionized water. Then, they were annealed at 400 °C for 4 h in air to get metal oxide phase.

## S2. Physical characterization of materials

**Scanning Electron Microscopy and Energy Dispersive Spectroscopy.** SEM analysis is
performed on a Zeiss LEO 1530 FE-SEM, operated at 5kV. The samples are directly observed as
grown on the GPR substrate with an In-lens SE detector. EDS analysis is performed employing an
Oxford 30 mm² SDD spectrometer, operating at 15kV. For XRD and XPS characterization, the
dense layer of NiCo₂O₄, Co₃O₄, and NiO nanostructures was removed from the graphite rod surface, collection both the graphite/clay mineral substrate and the produced metal oxide. **X-ray diffraction.** The crystallography of each material was investigated by the XRD (D8 Advance, Bruker) using CuKα radiation (λ= 1.54050 Å), 45 mA, 45 kV and (2θ) scale range from 10 to 85°. at a scan rate of 1° min⁻¹. Quantitative Rietveld analysis was used to quantify the relative distribution of each component in the composite system using HighScore Plus software [101, 102].

**X-ray photoelectron spectroscopy** spectra were collected on a Kratos Axis Ultra (Kratos Analytical, UK) spectrometer equipped with a monochromatic Al Kα (1486.6 eV). All spectra were recorded at a 90° takeoff angle. The high-resolution regions were acquired with 0.1 eV step and 20 eV pass energy (instrumental resolution better than 0.5 eV). All binding energy values were charge corrected to the C 1s signal (284.6 eV). Curve fitting was performed using a combined Gaussian and Lorentzian line profile after Shirley's background subtraction by Casa XPS software.

**Transmission electron microscopy** (TEM) observations were performed on a JEOL JEM ARM 200F-Cold FEG operating at 200 kV, equipped with spherical aberration (Cs) image and probe correctors. Electron energy loss spectroscopy (EELS) experiments were performed in scanning transmission electron microscopy (STEM) mode. The spectrometer was set to an energy dispersion of 0.25 eV/channel to collect O-K, Co-L2,3 and Ni-L2,3 edges. Supplementary X-maps were collected on an energy X-ray dispersive spectrometer (EDS) (JEOL Centurio). The oxide deposit was gently rubbed out of graphite rod before being deposited on a copper grid covered by a holey carbon film (200 mesh size).

**Electrochemical measurements.** In the typical three electrode assembly, a graphite rod and Ag/AgCl electrode filled with (3 M KCl) were used as the counter and reference electrodes, respectively. The grown NiCo₂O₄, Co₃O₄, and NiO nanostructures onto graphite pencil rod and bare pencil rod were used as direct electrodes for the OER characterization. The grown metal oxide nanostructures onto the graphite rods were covered with 5% Nafion membrane as binder to avoid the leakage of the nanostructured materials from the surface of graphite rod during the measurements. Then pencil graphite rod was attached with copper wire through parafilm and also covered the most part of pencil graphite rod except a diameter of 0.12 cm² was exposed to electrolytic solution. After that, the pencil graphite rod electrode was placed into the electrochemical cell where its terminal was clamped with copper wire thus it complete the cell
circuit for the electrochemical analysis. The OER curves were measured at a scan rate of 5 mV s⁻¹ using linear sweep voltammetry (LSV) in different electrolytic 0.1, 1.0, and 6.0 M solutions of KOH de-aerated by N₂. The purification of electrolyte (KOH) was ensured in order to avoid the effect of cation concentration on the electrochemical performance of working electrode by the reported method [103, 104]. The electrolyte purification was observed in a plastic cell in such a way the cell was remained untouched with the fluoropolymer material which could cause the contamination from the glassy parts of the cell. Prior to the LSV, cyclic voltammetry (CV) was used at a scan rate of 10 mV s⁻¹ to stabilize the electrode. The cyclic voltammetry was used at different scan rates for the calculation of electrochemical active surface area. The electrochemical impedance spectroscopy was performed at the frequency range of 100 kHz to 0.1 Hz, sinusoidal potential of 5 mV and OER onset potential. All OER polarization curves are reported without iR compensation and the electrochemical experiments were performed on Versa-potentiostat made in Netherland. The measured experimental potentials are converted into reversible hydrogen electrode (RHE) using Nernst equation.

<table><caption>**Table S1:** The quantitative distribution of various components in each composite system using High score plus analysis on XRD data</caption><thead><tr><th></th><th>SiO<sub>2</sub></th><th>Graphite</th><th>MgO</th><th>NiO</th><th>Co<sub>3</sub>O<sub>4</sub></th><th>NiCo<sub>2</sub>O<sub>4</sub></th><th>Total</th></tr></thead><tbody><tr><td>NiO-P</td><td>17.8</td><td>68.8</td><td>5.6</td><td>7.8</td><td>--</td><td>--</td><td>100</td></tr><tr><td>Co<sub>3</sub>O<sub>4</sub>-P</td><td>18.2</td><td>69.5</td><td>7.1</td><td>--</td><td>5.2</td><td>--</td><td>100</td></tr><tr><td>NiCo<sub>2</sub>O<sub>4</sub>-P</td><td>17.9</td><td>70.8</td><td>5.7</td><td>--</td><td>--</td><td>5.6</td><td>100</td></tr><tr><td>Pristine pencil graphite rod</td><td>18.9</td><td>73.8</td><td>7.3</td><td>--</td><td>--</td><td>--</td><td>100</td></tr></tbody></table>
<figure><img src="image_18.png" /><figcaption>**Figure S1:** SEM images of PGR at different magnifications.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Figure S2:** SEM images taken at different magnifications of the catalysts after their preparation: a) and b) NiO/PGR, c) and d) Co₃O₄/PGR and e) and f) NiCo₂O₄/PGR.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Figure S3:** SEM images taken at different magnifications of the catalysts after OER experiments: a) and b) NiO/PGR, c) and d) $Co_3O_4$/PGR and e) and f) $NiCo_2O_4$/PGR.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Figure S 4:** EDS analysis of a) PGR, and the prepared electrocatalysts: b) NiO/PGR, c) $Co_3O_4$/PGR and d) $NiCo_2O_4$/PGR.</figcaption></figure>
<table><caption>**Table S1:** Comparison of average elemental composition of GPR and the prepared electrocatalysts extracted from EDS spectra.</caption><thead><tr><th>at. %</th><th>GPR</th><th>NiO/PGR</th><th>Co<sub>3</sub>O<sub>4</sub>/PGR</th><th>NiCo<sub>2</sub>O<sub>4</sub>/PGR</th><th>NiO/PGR</th><th>Co<sub>3</sub>O<sub>4</sub>/PGR</th><th>NiCo<sub>2</sub>O<sub>4</sub>/PGR</th></tr><tr><th></th><th colspan="4">Before OER experiment</th><th colspan="3">After OER experiment</th></tr></thead><tbody><tr><th>C</th><td>76.5</td><td>19.4</td><td>35.6</td><td>32.5</td><td>12.3</td><td>22.9</td><td>24.5</td></tr><tr><th>O</th><td>17.3</td><td>22.3</td><td>41.2</td><td>38.2</td><td>44.5</td><td>49.5</td><td>19.4</td></tr><tr><th>Na</th><td>-</td><td>0.1</td><td>-</td><td>0.1</td><td>0.3</td><td>-</td><td>0.1</td></tr><tr><th>Mg</th><td>0.2</td><td>0.1</td><td>0.2</td><td>0.3</td><td>-</td><td>0.1</td><td>0.1</td></tr><tr><th>Al</th><td>1.5</td><td>1.0</td><td>0.9</td><td>1.2</td><td>1.0</td><td>0.6</td><td>0.8</td></tr><tr><th>Si</th><td>3.6</td><td>2.7</td><td>1.5</td><td>2.8</td><td>2.1</td><td>0.8</td><td>1.6</td></tr><tr><th>S</th><td>-</td><td>1.1</td><td>0.2</td><td>0.6</td><td>0.6</td><td>0.2</td><td>0.3</td></tr><tr><th>Ca</th><td>0.1</td><td>0.2</td><td>0.1</td><td>0.3</td><td>0.1</td><td>0.3</td><td>0.2</td></tr><tr><th>Fe</th><td>0.7</td><td>2.2</td><td>0.6</td><td>1.3</td><td>1.2</td><td>0.7</td><td>2.1</td></tr><tr><th>Co</th><td>-</td><td>-</td><td>19.6</td><td>13.4</td><td>-</td><td>24.9</td><td>34.1</td></tr><tr><th>Ni</th><td>-</td><td>51.0</td><td>-</td><td>9.5</td><td>37.9</td><td>-</td><td>16.8</td></tr></tbody></table>
<figure><img src="image_22.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>c</figcaption></figure>

<figure><img src="image_24.png" /><figcaption>d</figcaption></figure>

<figure><img src="image_25.png" /></figure>

**Figure S5:** XPS spectra (a) Ni 2p of NiO/PGR; (b) Co 2p of Co<sub>3</sub>O<sub>4</sub>/PGR; (c) Ni 2p and (d) Co 2p of NiCo<sub>2</sub>O<sub>4</sub>/PGR.
<figure><img src="image_26.png" /><figcaption>**Figure S6:** TEM images of NiCo₂O₄/PGR before OER experiments at different magnifications.</figcaption></figure>
<figure><img src="image_27.png" /><figcaption>**Figure S7:** X-maps from EDS analyses of NiCo₂O₄/PGR before (a-c) and after (d-f) the OER reactions.</figcaption></figure>
<figure><img src="image_28.png" /><figcaption>**Figure S8**, a.CV curves of bare graphite pencil at different scan rates in 1.0 M KOH, b. CV curves of NiO-P at different scan rate in 1.0 M KOH, c. CV curves of Co₃O₄-P at different scan rates in 1.0 M KOH, d. CV curves of NiCo₂O₄-P at different scan rates in 1.0 M KOH</figcaption></figure>
<figure><img src="image_29.png" /><figcaption>**Figure S9.** LSV curves of bare graphite pencil, NiO-P, Co₃O₄-P, NiCo₂O₄-P, at a scan rate of 5mVs⁻¹ in 0.1 M KOH</figcaption></figure>