Binary and Ternary 3D nanobundles Transition Metal Oxides Functionalized Carbon Xerogels as Electrocatalysts toward Oxygen Reduction Reaction

A series of carbon xerogels doped with cobalt, nickel and iron have been prepared through the sol-gel method. The doped carbon xerogels were further functionalized with binary and ternary transition metal oxides; namely nickel cobaltite, zinc cobaltite and zinc-nickel-cobalt oxide by hydrothermal method. A development in the mesopore volume is achieved for functionalized carbon xerogel doped with iron. However, in functionalization of carbon xerogel with ternary metal oxides a reduction in pore diameter and mesopore volume is found. In addition, all functionalized metal oxides/carbon are in the form of 3D nanobundles with different lengths and widths. The prepared samples have been tested as electro-catalysts for oxygen reduction reaction (ORR) in basic medium. The activity of functionalized cobalt doped carbon xerogel toward ORR is higher than that for functionalized carbon doped with iron or nickel. Among all the prepared samples being nickel cobaltite functionalized cobalt doped carbon xerogels (NiCo 2 O 4 /Co-CX) the best active electro-catalyst for oxygen reduction reaction with lowest onset potential of -0.06 V. The activity of NiCo 2 O 4 /Co-CX is attributed to its lower equivalent series resistance of 2.37  together with its higher affinity for ionic adsorption exhibited from electrochemical impedance spectroscopy study. catalysis 23 energy storage 24 and catalyst support in PEM fuel cell prepare nickel cobaltite decorated carbon xerogel composite (NiCo 2 O 4 /CX) for methanol electro-oxidation. The data shows a good catalytic current density of 98 mA.cm ─2 at 0.29 V with stability reaches 90.6% after 100 repetitive cycles.


Introduction
The negative effects that fossil combustion has had upon the environment has created a demand to develop and implement alternative clean energy sources [1][2] . Fuel cells can dramatically decrease the greenhouse gas emissions because it is considered as zero carbon emissions device.
Proton exchange membrane (PEM) fuel cell is the most widely used fuel cell in potential applications such as vehicles and power units [3][4] . Unfortunately, the high cost per unit energy delivered reduces the use of PEM fuel cell in a large scale and introducing to market. This problem can be solved either by reducing the used fuel cost, typically H2, or searching for electrocatalyst that enhance the overall efficiency and energy generation.
The fuel cell half reactions are anodic hydrogen oxidation reaction (HOR) and cathodic oxygen reduction reaction (ORR). The overpotential for HOR is considered as negligible while for ORR it requires high energy to initiate the reaction due to large kinetic inhibition. Thus, the oxygen reduction reaction (ORR) is considered as the rate determining step and reducing its overpotential will enhance the fuel cell efficiency. It was found that the ORR can be carried out through four electron transfer pathway or two electron transfer pathway (see below). The four electron transfer pathway is the favored one because it forms the hydroxyl species rather than peroxide 5-6 that causes membrane degradation and electrode fouling. The ORR reactions in both acidic and basic medium can be summarized as follows 3,7 .
Acidic media O2 + 4H + + 4 e ─ 2H2O direct (4 electron pathway) O2 + 2H + + 2e ─ H2O2 indirect (2 electron pathway) (2) H2O2 + 2H + + 2e 2H2O Alkaline media O2 + 2H2O + 4e 4OH ─ direct (4 electron pathway) O2 + H2O + 2e HO2 ─ + OH ─ indirect (2 electron pathway) (5) HO2 ─ + H2O + 2e 3OH ─ (6) The peroxide formation can be avoided by using an effective electrocatalyst that force the reaction towards the four electron pathway. Platinum (Pt) and its alloys are considered as the best electocatalysts for ORR that makes the reaction carried out through 4 electron pathway with the formation of only water as the product [8][9][10] . However, the platinum high cost and scarcity reduces its global use on a large scale on PEM. Recently, transition metal oxides based electrocatalysts, whether it is mono-metal, binary-metal, or ternary-metal oxides, become promising candidates for Pt-based materials in ORR application due to their active sites, high electronic conductivity, low cost and environmental benignity [11][12] . But, the activity of transition metal oxides toward ORR still unsatisfactory due to some reasons such as less electronic conductivities than Pt ones 13 . Improving conductivity of transition metal oxides can be achieved through doping with electron donor or by their encapsulation in carbon matrix [14][15] . Jie Wang et.al, synthesized an electrocatalyst of hollow structured carbon supported nickel cobaltite nanoparticles for oxygen reduction and evolution reactions 16 . In this work, vulcan XC-72 was used as the carbon source and they found that the hollow structure of nickel cobaltite increases the number of active sites which in turn increases the contact between the electrolyte and the catalyst results in improving the ORR activity.
Recently, carbon gel [17][18][19] which is a novel carbon nanomaterial, finds applications in wide research areas due to its tunable surface area and porosity depending on the synthesis conditions [20][21][22] . Carbon gels in its of both forms, aerogel and xerogel, were used in different fields such as catalysis 23 , energy storage 24 , and catalyst support in PEM fuel cell 25  were prepared, to investigate the role of carbon support in ORR application. The doped carbon xerogels were prepared through the sol-gel method from resorcinol and formaldehyde monomers. After that, the doped carbon xerogel were further functionalized with binary and ternary transition metal oxides, i.e, nickel cobaltite (NiCo2O4), zinc cobaltite (ZnCo2O4) and zinc nickel cobalt oxide (Zn-Ni-Co oxide), by the hydrothermal process to evaluate the role of metal oxides. The prepared materials will be tested as electrocatalysts to oxygen reduction reaction in basic medium.

Preparation of metal doped carbon xerogels (M-CX)
Metal doped carbon xerogels were prepared from resorcinol and formaldehyde monomers through the sol-gel method published elsewhere 28 . Briefly, resorcinol (R) and formaldehyde (F) were mixed together with a molar ratio of 1:2 in presence of water (W) as solvent and metal acetate salt as the dopant (C). The molar ratio between R/W is 1:17 and the amount of metal dopant (M = Co, Ni and Fe) was calculated to be 1 wt.% of the carbon matrix. After gelation, the obtained organic gels were subjected to microwave drying process followed by carbonization in N2 atmosphere at 900 for two hours with a heating rate of 1 /min. minutes, then transferred into 100 ml Teflon lined stainless steel autoclave and the temperature was held at 180 for 12 hrs. After cooling down to room temperature, the powder was separated with centrifugation then washed several times with water and ethanol followed by drying at 60 for 12 hr. Finally, the sample was calcined in air at 360 for 3 hrs. Same procedure was followed for preparation of ZnCo2O4/Co-CX except with changing nickel precursor with zinc one. In addition, in preparation of ternary transition metal oxide/ carbon xerogel (Zn-Ni-Co oxide/ Co-CX) 0.5 mmol of zinc acetate, 0.5 mmol of nickel acetate, 1 mmol of cobalt acetate and 6 mmol of urea were mixed together and the same preparation steps were followed.

Characterization of the prepared materials
The surface areas and porosity of the prepared samples were characterized by N2-gas adsorption at -196 (Quadrasorb, Boynton Beach, Florida, USA). Samples morphology is evaluated with scanning electron microscopy (SEM), Zeiss SUPRA40VP. The samples particle size is determined by high resolution transmission electron microscopy (HRTEM), FEI Titan G2 60-300 microscope (FEI, Eindhoven, Netherlands). While samples crystallinity is characterized Xray diffraction (XRD) BRUKER D8 ADVANCE diffractometer (BRUKER, Rivas-Vaciamadrid, Spain). X-ray photoelectron spectroscopy (XPS) measurements were carried out with a physical electronics ESCA 5701 (PHI, Chanhassen, MN, USA) operating at 12 KV and 10 mA and equipped with a MgK X-ray source (հυ = 1253.6 eV).

Electrode preparation
The working electrode was prepared with dispersing 5 mg of the prepared materials in 400 µL isopropanol and 30 µL nafion solution (5wt.%), then sonicated for 10 minutes. 10 µL of the prepared ink were deposited onto rotating disk electrode (RDE) surface with 3 mm glassy carbon tip and dried under infrared lamp.

Electrochemical measurements
All electrochemical measurements are performed using biologic VMP multichannel potentiostat with standard three electrodes electrochemical cell in which Ag/AgCl electrode was used as reference while Pt-wire was used as counter electrode. 0.1 M KOH was chosen to be the used electrolyte. Cyclic voltammetry (CV) were carried out in N2 and O2 saturated electrolyte in a potential window ranging from ─0.8 to 0.4 V with two scan rates 50 mV•s ─1 and 5 mV•s ─1 .
While linear sweep voltammograms (LSV) were obtained in O2 saturated electrolyte with different rotation speeds (from 500 to 4000 rpm) at 5 mV•s ─1 . Data obtained from LSV were fitted to Koutecky-Levich model in order to determine the number of transferred electrons and the electro-catalytic performance of each sample as follows [29][30] .
Electrochemical impedance spectroscopy (EIS) was performed in two electrode system in order to evaluate the electrode resistance and the equivalent series resistance. The applied frequency was ranging from 100 KHz to 1 mHz.

Physicochemical Properties
The porous texture of the samples was characterized with N2 gas adsorption. Before analysis the samples were degassed at 100 for 12 hr. BET equation was applied to the obtained isotherms to calculate their corresponding surface areas. The data obtained from N2 gas adsorption are compiled in Table 1. In evaluating the porous texture of functionalized carbon support, the highest specific surface areas (SBET = 74 m 2 g ─1 ), micropore volumes (Wo = 0.03 cm 3 g ─1 ) and pore diameter (Lo = 3.23 nm) was found for functionalized iron doped carbon xerogel (NiCo2O4/Fe-CX) followed by cobalt then nickel. While the lowest surface area, mesopore volume (Vmeso) and pore diameter was obtained for ternary transition metal oxide functionalized carbon xerogel (Zn-Ni-Co/Co-CX). These data are also confirmed by applying the DFT equation to the adsorption part of N2 isotherms, Nickel cobaltite (NiCo2O4/Co-CX) and zinc cobaltite (ZnCo2O4/Co-CX) functionalized carbon xerogel, have comparable surface areas and pore diameter. However, a reduction in the mesopore volume and surface area of carbon xerogel doped with ternary metal oxide was found; Zn-Ni-Co oxide/Co-CX. The pore size distribution obtained from the DFT study is represented in Fig. 1. In which a development in the mesopore volume for carbon xerogel doped with iron.  The mean particle size for the prepared sample is calculated by applying Scherrer equation to the XRD patterns, Table 2. Nickel cobaltite functionalized carbon xerogel have similar particle size ranging between 21.1 25.5 nm. There is an increase in the particle size to 39.2 nm and 38.8 nm when carbon xerogel was doped with zinc cobaltite and Zn-Ni-Co oxide, respectively. nanobundles with different lengths [31][32] . The nanobundle structure of the prepared samples is also confirmed with TEM analysis Fig. 4, with homogeneously anchoring of nanobundles on the surface of carbon xerogels (Fig. 4a-e). In addition, the nanobundles have different widths which is wider in case of ZnCo2O4/Co-CX (Fig. 4d).       35 . Iron was not detected in any sample by XPS, and Fig. 5c shows the Zn2p spectra. The presence of zinc cobaltite, ZnCo2O4, is confirmed by a peak centered at 1020.7 eV 36 , while the peak at 1021.5 eV 1021.8 eV is assigned to ZnO 37 .  surface area and part of the oxygen content can be due to the calcination treatment during the preparation of the samples. Table 5. Surface chemical composition in wt. % obtain by XPS.

Oxygen reduction reaction activity
Samples performance in presence and absence of oxygen was studied using cyclic voltammetry Regarding studying the effect of the dopant metal (M = Co, Ni, Fe) on the activity of carbon xerogels toward ORR, the three doped samples functionalized with NiCo2O4 were tested as electrocatalysts for ORR, Fig. 6b. The results show that the electrochemical performance towards ORR is higher for cobalt doped carbon xerogels (Co-CX) followed by samples doped with iron (Fe) and nickel (Ni) which is in a good agreement with previous published work 28 In general incorporation of transition metal into the carbon structure promotes the catalytic activity for ORR due to the generation of more active sites 40   In order to study the effect of functionalized metal oxide, two binary transition metal oxides (i.e. NiCo2O4 and ZnCo2O4) and one ternary metal oxide (Zn-Ni-Co oxide) functionalized over cobalt doped carbon xerogel (Co-CX) were tested, Fig. 6c. Among all samples being NiCo2O4/Co-CX the one that has the highest electrochemical performance to ORR followed by Zn-Ni-Co oxide /Co-CX and ZnCo2O4/Co-CX. These results are also confirmed with the data obtained from Linear Sweep Voltammetry (LSV). The LSV for NiCo2O4/Co-CX was carried out using rotating desk electrode with different speeds (Fig. 7a). The estimated number of electrons transferred at a given potential obtained from Koutecky-Levich Equation are compiled in Table 6. The data obtained from LSV for the prepared electrocatalysts also confirms that, carbon xerogel doped with cobalt has the highest electrochemical performance to ORR (Fig. 7b) and NiCo2O4 functionalized carbon xerogel is the better electrocatalyst (Fig. 7c). These results can be explained to the higher number of electrons transferred in these both samples. For example, in NiCo2O4/Co-CX the oxygen reduction reaction occurs mainly through the 4e transfer ( Fig. 8a) with lower onset potential Eonset, Table 6. However, in NiCo2O4/Fe-CX and NiCo2O4/Ni-CX the oxygen reduction reaction occurs through a combination of both 2e and 4e , results in a value of n = 3.6 and 3.5, for both respectively. Likewise, in comparison between different metal oxides being NiCo2O4 the one that undergoes the oxygen reduction reaction via the 4e transfer (Fig. 8b). Besides, the number of electrons transferred has a direct relation and depend on the electrocatalyst resistance. The electrochemical impedance spectroscopy (EIS) is an important technique that determine the behavior of electrode in certain application and its resistance. Fig. 9 shows the Nyquist plots for the prepared electrodes. As it can be seen, a semicircle is observed at high frequency region due to charge transfer resistance on electrode/electrolyte interface. While at low frequency, a purely capacitive behavior is obtained and the combination between both resistive and capacitive behavior is known as Warburg region that has a relation to the diffusive resistance 31,43 . The equivalent series resistance of the electrode can be determined by the intersection of the semicircle with the real axis, Table 6.  Similar results obtained in comparing metal oxides activity to ORR, for NiCo2O4 and Zn-Ni-Co oxide the diameter of the semicircle is similar indicating lower resistance between electrode/ electrolyte interface. Although the ESR of Zn-Ni-Co oxide is lower than that for NiCo2O4 that indicating better electronic conductivity, the vertical line of the capacitive part for NiCo2O4/Co-CX is more perpendicular to the X-axis which means swift ion diffusion in electrolyte and good adsorption onto the electrode surface results in higher activity to ORR (Fig. 9b). In any case, we expected a better electro-catalytic behavior of our ternary metal oxide samples, incorporating Zn to the metal phase; however, this has not occurred at least in the desirable extension. One plausible explanation can be due to the important increasing of the metal particle size when Zn is added, which besides avoids a good comparison of the electro-catalytic activity among all the samples. The available bibliography about a possible effect of the metal particle sizes on the ORR activity is not clear. Some works with Pt supported on carbon materials found a relation between the particle size and the specific activity towards ORR, however while some of them mention that for oxygen reduction a loss of catalytic activity with the decreased Pt particle size occurs due to the stronger adsorption of oxygenated species 10, 44 , other authors found an optimal Pt particle size 45 . Nevertheless, with Fe, Co or Ni based catalysts the previous mentioned tendencies seem to be different, where the small sized cobalt nanoparticles and dispersion in carbon provide fast electron transport between the carbon matrix and the cobalt nanoparticles, leading to efficient electrical conductivity 11,46 . This explanation is also in agreement with our results where the extremely large main crystallite sizes of samples containing Zn, in relation with the free Zn samples, can not only justify their comparative electro-catalytic behavior but also show them as very attractive active phase for ORR, whose metal sizes should be better controlled and be optimized for future applications.

Conclusions
NiCo2O4, ZnCo2O4, and Zn-Ni-Co oxide nanobundles were homogeneously developed on the carbon xerogel surface by hydrothermal method. The type of carbon xerogel dopant plays an important role in ORR activity being cobalt doped carbon xerogels the one that has higher electrochemical activity to ORR than Fe or Ni. The NiCo2O4/Co-CX nanobundle has excellent activity to oxygen reduction reaction. This activity comes from its lower equivalent series resistance which increases the number of ions reaching the electrode surface beside good adsorption of electrolytic ions onto its surface. The lower onset potential for NiCo2O4/Co-CX together with its higher current densities leads that NiCo2O4/Co-CX nanobundles to be very attractive electrodes for ORR. Despite Zn-Ni-Co oxide/Co-CX has the lowest equivalent series resistance indicating higher electronic conductivity; its reduction in the mesopore volume could be able to reduce the ionic diffusion and adsorption decreasing its activity for ORR. However, the extremely large main crystallite sizes of samples containing Zn, in relation with the other samples, avoid a good comparison of the electro-catalytic activity among of the samples. The role of Zn in this type of transition metal oxides nanobundles based ORR catalysts cannot be underestimated, and a future work in which this parameter can be better controlled is necessary to obtain coherent conclusions.