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

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


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][2][3][4][5][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][10][11][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][14][15][16][17]. The spinel oxides composed of transition metals like Co [18][19][20][21][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][31][32][33]. Beside this, bimetallic oxides of cobalt having a spinel structure were investigated in the wide range of energy storage and conversion applications [34][35][36]. Among them, nickel-cobalt bimetallic oxides of NiCo2O4 have been intensively studied in different electrochemical applications in the recent past [37][38][39][40][41][42][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][45][46][47][48][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 4 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][52][53][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 Co 2+ 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][59][60][61][62][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 stgudy. 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. Howeve, the use of PGR as a substrate and co-catalyst has strongly enhanced the electrocatalytic properties of NiO, Co3O4 and NiCo2O4 electrocatalysts. Importantly, the microporosity of modified PGR has enabled the easily bubbling of O2 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 Co3O4, NiCo2O4 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, Co3O4/PGR, and NiCo2O4/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, Co3O4-P, and NiCo2O4-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 NiCo2O4 electrode surface has favored the efficient oxidation/reduction reaction by producing the intense bubbling of O2 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. 6 Scheme 1 Graphical view of the prepared metal oxide/PGR composite electrodes and their use for OER process

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.  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] 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 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 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/Po = 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.

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 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 Ni 2+ /Ni 3+ and Co 3+ /Co 2+ 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 Co 3+ in the cobalt based catalysts can enhance the OER because Co 3+ ions provide a high probability for the adsorption of electrophilic species and consequently they facilitate of oxygen in the OHions 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 m 2 g −1 and 1.802 cm 3 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 Co 3+ 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]. 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 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 6.0M KOH was highly enhanced 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.

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, 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.     of NiCo2O4-P before (a-f) and after the OER reactions (g-l) (P stands for PGR).     This work [    purchased from Sigma Aldrich Karachi, Pakistan. These chemicals were of analytical grade and used without any further purification.  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 references 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 cm 2 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