Synthesis and characterization of NiCr2O3-CeO2NRs anode electrocatalyst for ethanol electrooxidation reaction in alkaline media

In this paper, ceria (CeO2) nanorod (NR) supported Ni-Cr2O3 anode electrocatalysts were synthesized as nonnoble metal-based anode electrocatalysts for ethanol electrooxidation reaction (EOR) in alkaline media. Physicochemical characterization of the electrocatalysts was investigated by XRD, SEM, and TEM techniques. Electrochemical performances of the catalysts were investigated via cyclic voltammetry (CV), linear sweep voltammetry (LSV), and chronoamperometry (CA) techniques. The data from linear LSV were used for the diffusion coefficient of the electrocatalysts. The CA experiments’ results showed the tolerance for catalytic poisoning and durability of the synthesized electrocatalysts.

potential as a cocatalyst for alcohol electrooxidation processes [19,20].Unfortunately, the lack of studies with Cr 2 O 3 is a gap in the literature therefore, this gap must be filled with valuable studies.
CeO 2 is amongst the most significant cocatalysts.Because of its good oxyphilic properties, CeO 2 is known to aid electron transport during electrochemical processes.It has been documented in the literature that anode catalysts including CeO 2 produce a high current density during the electrooxidation of alcohols and promote a negative onset potential shift.As known, the shape and size of nanoparticles highly affect the activity of the catalysts they form.Thus, the morphology of CeO 2 has been proven in the literature to affect catalyst interactions.CeO 2 nanorods are high aspect ratio nanomaterials and these nanorods have been shown to have greater oxygen vacancy density and catalytic activity for processes such as EOR [21][22][23][24][25].
According to the literature research, it is known that transition metal oxides will increase the electrocatalytic activity of Ni thus, this issue should be emphasized more.To our knowledge, there is no study in the literature that uses NiCr 2 O 3 -CeO 2NRs electrocatalysts for EOR in alkaline media.Accordingly, the aim of this study is to find the most suitable weight ratio for ethanol electrooxidation performance by synthesizing NiCr 2 O 3 -CeO 2NRs electrocatalysts in different weight ratios using Cr 2 O 3 and CeO 2 nanorods as cocatalyst and support materials.Nickel-containing catalysts were synthesized by the modified polyol method and their physicochemical characterizations were investigated by XRD, SEM-EDS, and TEM techniques.The electrochemical performances of the catalysts were observed by CV, LSV, and CA methods.

Preparation of the catalysts
Cr 2 O 3 NPs were synthesized via the thermal decomposition method [26]: The required amount of Cr(NO 2 ) 3 .9H 2 O is mixed with DI water and stirred at room temperature.The solution was dried at 110 °C for 12 h and then calcinated at 800 °C (heating rate 5 °C min -1 ) for 5h.
CeO 2 NRs were prepared as follows [27]: 3 g of Ce(NO 3 ) 3 .6H 2 O were dissolved in DI water.This solution is mixed with the required amount of 11.86 M NaOH solution (70 mL) for 30 min and then, poured into an autoclave at 100 °C for 24 h.Next, the final solution was cooled down to room temperature naturally, filtered, washed with DI water, and dried at 65 °C overnight.
Ni-Cr 2 O 3 /CeO 2NR electrocatalysts were synthesized via a modified polyol method [28]: Required amounts of 0.1 M NiCl 2 .6H 2 O in ethylene glycol (EG), EG, Cr 2 O 3, and CeO 2 NRs were mixed for a while then the mixture was heated to 120 °C in an oil bath.Next, a proper amount of hydrazine hydrate (HH) was added and mixed for 2 min.Lastly, 0.5 M NaOH (in EG) was added and stirred.The reaction is completed with the formation of black participate.
Preliminary experiments were carried out to determine the catalyst combinations given in Table 1.As a result of the preliminary experiments, the weight range used in the catalyst combinations was determined in Table 1.The aim here is to find the weight ratio of the catalyst with the highest EOR performance among the prepared electrocatalysts.Each catalyst was desired to contain 20% wt.CeO 2 NR and %80 wt.NiCr 2 O 3 .In addition, it is planned that the NiCr 2 O 3 portion of each catalyst will have different weight ratios of Ni and Cr 2 O 3 NPs.Accordingly, NiCr 2 O 3 -1, NiCr 2 O 3 -2, and NiCr 2 O 3 -3 have 95%, 90%, and 85% Ni NPs by weight, respectively.

Electrochemical characterization
The electrocatalytic activity of the NiCr 2 O 3 -CeO 2NRs electrocatalysts towards ethanol oxidation reaction (EOR) was performed by CHI 1100C Potentiostat using a glassy carbon electrode (GCE) with an area of 0.07cm -2 as working electrode, a Pt wire as the counter electrode, and lastly an Ag/AgCl electrode as a reference electrode.Nickel-based anode electrocatalysts were activated in 0.1M KOH solution at 0.01 Vs -1 scan rate.CV, LSV, and CA experiments were performed in a solution containing 0.05M KOH and 0.5M CH 3 CH 2 OH.

Electrochemical characterization results
Catalyst ink was prepared and applied to a glassy carbon electrode (GCE) for electrochemical analysis.To prepare the ink 200 μL of isopropyl alcohol, 10 μL of Nafion solution (5 wt%), and 1 mg of catalyst were ultrasonicated for 30 min.Also, the surface area of the GCE is 0.07 cm 2 and in order to load 1 mg cm −2 catalyst, 15 μL of the catalyst ink was attached to the GCE.The GCE's surface was cleaned with Al 2 O 3 paste, washed with DI water, and dried under N 2 gas before the ink was deposited.Then, using CHI 1100C Potentiostat, electrochemical data were measured.Before each experiment, N 2 gas was circulated through the solutions.
CV results are shown in Figure 5 for the synthesized electrocatalysts.CV voltammograms of the electrocatalysts were obtained at a scan rate of 0.01 V s -1 for electrolytes containing 0.1M KOH and 0.05M KOH+0.5MEthanol.
The onset potential and current density are the two prominent parameters that characterize the activity of electrocatalysts.Complete ethanol oxidation to CO 2 is a necessary condition for achieving the highest current.Figure 5a indicates that in the absence of ethanol, two redox peaks at 580 mV (anodic peak) and 450 mV (cathodic peak) were seen on the synthesized catalysts.These peaks are attributable to the nickel hydroxide (Ni(OH) 2 )/nickel oxo hydroxide (NiO(OH)) transition [31].
A rise in current density and/or the occurrence of an anodic peak indicating ethanol electrooxidation are two essential signs of the electrocatalyst's activity towards ethanol electrooxidation.With the addition of ethanol to the blank media, the obtained current density for each of the synthesized catalysts was enhanced.Depending on the weight percentage of Cr 2 O 3 NPs were present in the structure, the peak intensity varied.Consequently, introducing Cr 2 O 3 NPs increased the synergistic effect and showed broad oxidation peaks.
Among the synthesized catalysts illustrated in Figure 5b, the NiCr 2 O 3 -CeO 2NRs -2 electrocatalyst has the highest current density.Additionally, the catalysts' onset potentials were examined.As a consequence, adding more Cr 2 O 3 NPs until a proper amount caused the onset potentials to move negatively, indicating better catalytic activity.This increase in current density confirmed findings that the amount of Cr 2 O 3 NPs and Ni seeks to enhance EOR.Because the amount of Cr 2 O 3 NPs in the catalyst is crucial for maximizing the synergistic effect since too many Cr 2 O 3 NPs may obstruct the catalyst's active sites and reduce the synergistic effect, which will ultimately affect the electrocatalyst's activity and durability.In conclusion, NiCr 2 O 3 -CeO 2NRs electrocatalysts are beneficial at EOR, and the suitable participation of Cr 2 O 3 has increased anodic peak current density.As can be observed from the CV tests, the electrocatalysts' various metallic nickel weight percentages mainly generated diverse catalytic activity.It is noticeable that the catalytic activity is accelerated with a cocatalyst, even if an increase in the quantity of metallic nickel in the electrocatalyst structure does not imply an enhancement in the electrocatalytic activity.It was noticed that the amount of metallic nickel had greatly changed the catalytic activity of the synthesized catalysts and that there was a link between the maximum current density and the amount of metallic nickel in the catalyst.
The electrocatalyst's overpotential, or the point at which the current density increases as a result of EOR, is known as the onset potential.It is crucial for an effective anode electrocatalyst to have a low onset potential and high current density since a superior electrocatalyst may be detected with lower overpotential.The overpotential is reduced when the cocatalyst is added to the electrocatalyst structure.Accordingly, the onset potentials and maximum current densities of the NiCr 2 O 3 -CeO 2NRs -1, NiCr 2 O 3 -CeO 2NRs -2, and NiCr 2 O 3 -CeO 2NRs -3 are given in Table 2.
Anodic Tafel plots of the synthesized catalysts are given in Figure 6.To derive the Tafel plots, the LSV technique was used, and the curves were recorded at a sweep rate of 1 mV s −1 in 0.05 M KOH + 0.5 M ethanol solution.The Tafel curves presented in Figure 6  Figure 7 displays the catalysts' LSV experiment results.Greater current density resulted from a higher scan rate, and this correlation enables us to determine the catalysts' diffusion coefficients.The diffusion coefficients are determined using (1) Here, the i p is peak current density, n is the number of electrons transferred (2.8), F is Faraday constant, A is electrodes geometric surface area, C is concentration, v is scan rate, D is diffusion coefficient, R is universal gas constant and T is temperature [33].
The role of Cr 2 O 3 NPs in advancing the Ni catalyst toward the electrooxidation process is supposed to be well addressed by the calculation of several kinetic parameters.Accordingly, the diffusion coefficient is one of the most important parameters.NiCr The data of CA testing to examine the catalysts' endurance are shown in Figure 8.Since the CHI 1100C potentiostat only permits 1000s for CA testing, the time frame was 1000 s.Chronoamperometry tests for the synthesized catalysts were conducted and the results are given in Figure 8.It is clear from the CV tests that the ErCi (Reversible Electron Transfer Followed by an Irreversible Homogeneous Chemical Reaction) mechanism may be involved in the EOR process [36].As a result, under that scenario, undesirable intermediates may occur; these intermediates may even be toxic.Since it is common knowledge that poisoning intermediates can reduce catalytic activity when they are adsorbed.Therefore, the antipoisoning capability of a high-performance catalyst is crucial.Different metals and oxides may be used in catalysts to avoid catalyst poisoning.For instance, Wang et al., synthesized Pt-MO x /C (M= Ce, Zr) electrocatalysts and conducted a CA experiment to investigate the stability of the synthesized electrocatalysts for 1000 s in 0.1M H 2 SO 4 + 1.0M C 2 H 5 OH solution.Accordingly, they reported that the stability of Pt-CeO 2 -ZrO 2 (Ce:Zr = 2:1) is the best among the synthesized catalysts [37].Another study was made by Chai et al., and they prepared Ir 3 Sn-CeO 2 /C electrocatalyst for EOR.Their CA results indicated that the stability of Ir 3 Sn-CeO 2 /C is better than Ir 3 Sn/C, Ir/C, and Pt/C [38].Accordingly, the stability of NiCr 2 O 3 -CeO 2NRs -2 is better than the NiCr 2 O 3 -CeO 2NRs -1 and NiCr 2 O 3 -CeO 2NRs -3.These results are coherent with the CV and LSV tests.Therefore, it can be concluded that the resistance to catalytical poisoning of NiCr 2 O 3 -CeO 2NRs -2 is better among the synthesized catalysts.On the other hand, it was seen that the stability of NiCr 2 O 3 -CeO 2NRs -3 is very poor.It can be a result of covered active sites of the catalysts or agglomerated particles.Accordingly, the importance of the catalyst composition is proven.

Conclusion
NiCr 2 O 3 -CeO 2NRs anode catalysts were prepared successfully, physicochemical, and electrochemical characterizations of the catalysts were investigated.The effects of Cr 2 O 3 NPs added to the structure of the catalyst on the current density were observed.The intensity of the peaks observed during the electrochemical characterizations changed depending on the weight percent of the Cr 2 O 3 NPs in the structure and the NiCr 2 O 3 -CeO 2NRs -2 electrocatalyst had the highest current density.When the onset potentials of the synthesized catalysts were examined, it was reported that adding an appropriate amount of Cr2O3 NP showed better catalytic activity by causing the onset potentials to shift in the negative direction.

Acknowledgement
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
), (331), and (420) are shown by the peaks at 2 = 28.60,33.14, 47.55, 56.39, 59.16, 69.49, 76.79, 79.14, respectively[30].Cr 2 O 3 peaks are more prominent than CeO 2 peaks.Figure1shows the XRD findings of NiCr 2 O 3 -CeO 2NRs electrocatalysts.All of the components' peaks are clearly selectable, and there are no peaks visible outside of them.As a result, binary catalysts may plainly be observed to be effectively synthesized.The SEM results of the Ni NPs, Cr 2 O 3 NPs, CeO 2 NRs, and NiCr 2 O 3 -CeO 2NRs electrocatalysts are given in Figure 2. Characteristic structures of the Ni NPs, Cr 2 O 3 NPs, CeO 2 NRs can be selected clearly from Figure 2a-2c and therefore, it can be concluded that the Ni NPs, Cr 2 O 3 NPs, CeO 2 NRs are well-synthesized.Figure 2d-2f represents the SEM results of NiCr 2 O 3 -CeO 2NRs electrocatalysts.It is seen that the surface of the NiCr 2 O 3 NPs was covered with CeO 2 NRs well.The composition of the synthesized catalysts was investigated via EDS analysis, and the results are displayed in Figure 3. Here, it is seen the EDS results of the NiCr 2 O 3 -CeO 2-NRs -1, NiCr 2 O 3 -CeO 2-NRs -2, NiCr 2 O 3 -CeO 2-NRs -3.The corresponding elements namely, Ni, Cr, Ce, and O can be detected in the prepared samples.Accordingly, the metallic Nickel atomic ratio of the NiCr 2 O 3 -CeO 2-NRs -1, NiCr 2 O 3 -CeO 2-NRs -2, NiCr 2 O 3 -CeO 2-NRs -3 was determined as 95.66%, 91.23%, and 81.33%, respectively.
showed that NiCr 2 O 3 -CeO 2NRs -2 had the lowest slope values (3.56 V dec −1 ), implying a lower overpotential and better reaction kinetics for oxidizing ethanol among the synthesized catalysts.The NiCr 2 O 3 -CeO 2NRs -2 surface's electron-transfer kinetics for EOR are better compared to other electrocatalysts according to the lower Tafel slope values.The Tafel slope's decrease suggests an improvement in the EOR's kinetics and could possibly predict an earlier C-C bond cleavage.The transfer coefficient (a) of NiCr 2 O 3 -CeO 2NRs -1, NiCr 2 O 3 -CeO 2NRs -2, and NiCr 2 O 3 -CeO 2NRs -3 for electrooxidation of ethanol was determined to be 0.816, 0.925, and 0.875, respectively where the Tafel slope being equal to n(1-a)F/2.303RT [32].

Table 1 .
Weight ratios of the components in catalyst combinations.