Fabrication of Metal (Cu and Cr) Incorporated Nickel Oxide Films for Electrochemical Oxidation of Methanol

Methanol electrochemical oxidation in a direct methanol fuel cell (DMFC) is considered to be an efficient pathway for generating renewable energy with low pollutant emissions. NiO−CuO and Ni0.95Cr0.05O2+δ thin films were synthesized using a simple dip-coating method and tested for the electro-oxidation of methanol. These synthesized electrocatalysts were characterized by X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS), and Raman spectroscopy. Different electrochemical techniques were used to investigate the catalytic activity of these prepared electrocatalysts for methanol oxidation, including linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and chronoamperometry (CA). In the presence of 0.3 M methanol, the current densities of NiO−CuO and Ni0.95Cr0.05O2+δ thin films were found to be 12.2 mA·cm−2 and 6.5 mA·cm−2, respectively. The enhanced catalytic activity of NiO−CuO and Ni0.95Cr0.05O2+δ thin films may be a result of the synergistic effect between different metal oxides. The Chronoamperometry (CA) results of the mixed metal oxide thin films confirmed their stability in basic media. Furthermore, the findings of electrochemical impedance spectroscopy (EIS) of mixed metal oxide thin films demonstrated a lower charge transfer resistance as compared to the pure NiO, CuO, and


Introduction
Due to the rapid increase in worldwide energy consumption and detrimental environmental emissions from conventional fossil fuel, explorations of alternative clean energy resources and economical devices for efficient energy conversions are demanded [1][2][3][4].
To manage these problems, direct methanol fuel cells (DMFCs) have attracted extensive research attention, owing to the benefits they provide of high energy conversion efficiency, low pollution emissions, high energy density, easy fuel storage, and ambient operating conditions [5]. Pt-based catalysts were commonly used in DMFCs for methanol oxidation [6,7]. Even though Pt and Pt-based catalysts are widespread and effective for catalysis, they still present significant challenges such as their high cost, restricted availability, low stability, Germany. The reagents were of analytical grade and used without any further treatment. Deionized water was used for both synthesis and treatment under atmospheric conditions.

Synthesis of Mixed Metal Oxide Thin Films
The NiO−CuO and Ni 0.95 Cr 0.05 O 2+δ thin films were prepared on an FTO glass substrate using a simple dip-coating method. For a typical reaction 0.1 M Ni, Cu, and Cr precursor solutions were prepared by dissolving (Ni(CH 3 COO) 2 ·4H 2 O), (Cu(CH 3 COO) 2 ), and (Cr(C 5 H 7 O 2 ) 3 ), respectively, in a 5 mL mixture of methanol and ethanol (1:1) followed by stirring for 15 min. The FTO glass substrates (2 × 1 cm 2 ) were washed thoroughly with detergent followed by ultrasonic cleaning in a 1:1 solution of ethanol and acetone for 20 min and then dried in air at room temperature before coating in precursor solutions. To prepare the NiO−CuO thin-film, the dried FTO glass substrate was first immersed in the Ni precursor solution for 30 s. After 30 s, it was removed from the precursor solution and dried by heating at 90 • C for 10 min followed by immersion in the Cu precursor solution for 30 s and heating at 90 • C for 10 min. This process of alternative dip-coating was repeated four times and finally, washing was performed with distilled water. Ni 0.95 Cr 0.05 O 2+δ thin films were also prepared by the same alternative dip-coating of FTO glass substrate in 0.1 M Ni and Cr precursor solutions. Finally, coated NiO−CuO and Ni 0.95 Cr 0.05 O 2+δ thin films were calcined in a muffled furnace at a temperature of 500 • C for 3 h. The overall scheme is available in Figure 1. The FTO glass substrate was preferred over glass carbon electrode (GCE) for direct coating of the catalyst from precursor via dip coating. Because, after dipping into the precursor solution, the FTO glass substrate was annealed at 500 • C, the catalyst film strongly adhered to the substrate, and an extra binder was not required for coating purpose. However, dip-coating from the precursor solution, followed by heating above 400 • C is not possible for the glassy carbon electrode (carbon material in presence of O 2 above 400 • C may decompose). Furthermore, when a powdered sample is loaded on GCE the adhesion is very poor and leaves the surface of the substrate during catalytic activity. The addition of a binder (nafion) provides better adhesion, however, the binder obstructs the flow of the electron from the catalyst to the GCE, which leads to poor catalytic performance.

Synthesis of Mixed Metal Oxide Thin Films
The NiO−CuO and Ni0.95Cr0.05O2+δ thin films were prepared on an FTO glass substrate using a simple dip-coating method. For a typical reaction 0.1 M Ni, Cu, and Cr precursor solutions were prepared by dissolving (Ni(CH3COO)2·4H2O), (Cu(CH3COO)2), and (Cr(C5H7O2)3), respectively, in a 5 mL mixture of methanol and ethanol (1:1) followed by stirring for 15 min. The FTO glass substrates (2 × 1 cm 2 ) were washed thoroughly with detergent followed by ultrasonic cleaning in a 1:1 solution of ethanol and acetone for 20 min and then dried in air at room temperature before coating in precursor solutions. To prepare the NiO−CuO thin-film, the dried FTO glass substrate was first immersed in the Ni precursor solution for 30 s. After 30 s, it was removed from the precursor solution and dried by heating at 90 °C for 10 min followed by immersion in the Cu precursor solution for 30 s and heating at 90 °C for 10 min. This process of alternative dip-coating was repeated four times and finally, washing was performed with distilled water. Ni0.95Cr0.05O2+δ thin films were also prepared by the same alternative dip-coating of FTO glass substrate in 0.1 M Ni and Cr precursor solutions. Finally, coated NiO−CuO and Ni0.95Cr0.05O2+δ thin films were calcined in a muffled furnace at a temperature of 500 °C for 3 h. The overall scheme is available in Figure 1. The FTO glass substrate was preferred over glass carbon electrode (GCE) for direct coating of the catalyst from precursor via dip coating. Because, after dipping into the precursor solution, the FTO glass substrate was annealed at 500 °C, the catalyst film strongly adhered to the substrate, and an extra binder was not required for coating purpose. However, dip-coating from the precursor solution, followed by heating above 400 °C is not possible for the glassy carbon electrode (carbon material in presence of O2 above 400 °C may decompose). Furthermore, when a powdered sample is loaded on GCE the adhesion is very poor and leaves the surface of the substrate during catalytic activity. The addition of a binder (nafion) provides better adhesion, however, the binder obstructs the flow of the electron from the catalyst to the GCE, which leads to poor catalytic performance.  These fabricated NiO−CuO, Ni 0.95 Cr 0.05 O 2+δ thin films were subjected to different physical and chemical characterizations. The phase and crystal structure of synthesized NiO−CuO, Ni 0.95 Cr 0.05 O 2+δ thin films were assessed by X-Ray diffractometer (BRUKER D8 Advance XRD, Karlsruhe, Germany) equipped with Cu Kα radiation of λ = 1.540608 Å and operated at 2θ (20-80 • ). X-ray photoelectron spectroscopy (XPS) (Versa probe II, ULVAC-PHI, Inc. Chanhassen, MN, USA) studies were conducted under an ultra-high vacuum (~10 −10 mbar). Surface morphology and elemental compositions of thin films were observed by FESEM (JEOL JSM-7600F, Tokyo, Japan) fitted with Oxford energydispersive X-ray spectroscopy (EDS, High Wycombe, UK). An atomic force microscopy (Nanosurf C3000 AFM, Nanosurf, Liestal, Switzerland) technique [AFM-NT-MDT] was used in a non-contact mode on all the fabricated films to study the surface topography and roughness using the attached NT-MDT software. Raman spectra were recorded under ambient conditions using Raman micro-spectroscopy (uRaman-532 TEC-Ci, Technospex, Singapore), equipped with a 532 nm laser and an optical microscope (Nikon Eclipse Ci L, Singapore).
Electrochemical studies of methanol oxidation were performed on GAMRY G750 (Gamry Instruments, Inc. Warminster, PA, USA) in the potentiostatic mode by using three testing techniques, including linear sweep voltammetry (LSV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). The NiO−CuO and Ni 0.95 Cr 0.05 O 2+δ thin films operated as a working electrode, with a platinum wire that served as a counter electrode and Ag/AgCl, KCl (3 M) as a reference electrode. A supporting electrolyte was used, namely, 0.5 M NaOH. The activity of thin films for methanol electro-oxidation was investigated using an active area of 1 cm 2 .

X-ray Diffraction
The powder X-ray diffraction measurements were used to examine the crystallographic structures and phase details of thin films. Figure

X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) experiments were carried out to investigate the chemical composition and chemical bonding state of elements in the NiO−CuO and

X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) experiments were carried out to investigate the chemical composition and chemical bonding state of elements in the NiO−CuO and Ni 0.95 Cr 0.05 O 2+δ thin films, shown in Figures 3 and 4. In a wide scan XPS spectrum of NiO−CuO (Figure 3a), the presence of strong peaks at binding energies of 285.6, 530.4, 855.2, and 933.6 eV corresponds to C, O, Ni and Cu [37], respectively. Carbon is common in the XPS analysis due to contaminations from both the inside and outside of the vacuum chamber. As shown in Figure 3b, the deconvoluted peaks of the binding energy of 855 and 857 eV belong to Ni 2p 3/2 , and the satellite peak at 861.6 eV reveals the existence of NiO [38,39]. In Figure 3c, the deconvoluted peaks at 933.9 and 935.134 eV are attributed to Cu 2p 3/2 , and confirmed by the presence of their satellite peaks at 941.7 and 943.3 eV [32]. The O 1s spectra (Figure 3d) has two distinct peaks at the binding energies of 530.9 and 532.9 eV. The binding energy peak at 530.9 eV corresponds to Ni-O and Cu-O bonds. While the peak at 532.9 eV is a result of the residual water and absorbed oxygen species such as hydroxyl or carbonate species on surface [40]. The wide scan XPS spectrum of

X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) experiments were carried out to investigate the chemical composition and chemical bonding state of elements in the NiO−CuO and Ni 0.95 Cr 0.05 O 2+δ thin films, shown in Figures 3 and 4. In a wide scan XPS spectrum of NiO−CuO (Figure 3a), the presence of strong peaks at binding energies of 285.6, 530.4, 855.2, and 933.6 eV corresponds to C, O, Ni and Cu [37], respectively. Carbon is common in the XPS analysis due to contaminations from both the inside and outside of the vacuum chamber. As shown in Figure 3b, the deconvoluted peaks of the binding energy of 855 and 857 eV belong to Ni 2p 3/2 , and the satellite peak at 861.6 eV reveals the existence of NiO [38,39]. In Figure 3c, the deconvoluted peaks at 933.9 and 935.134 eV are attributed to Cu 2p 3/2 , and confirmed by the presence of their satellite peaks at 941.7 and 943.3 eV [32]. The O 1s spectra (Figure 3d) has two distinct peaks at the binding energies of 530.9 and 532.9 eV. The binding energy peak at 530.9 eV corresponds to Ni-O and Cu-O bonds. While the peak at 532.9 eV is a result of the residual water and absorbed oxygen species such as hydroxyl or carbonate species on surface [40]. The wide scan XPS spectrum of        Figure S1). It is observed in all these SEM images that the substrate surface is evenly coated with dense and wellinterconnected nano-particles. Furthermore, in the case of NiO−CuO, several nanoparticles appear to be aggregated. The average particle size of NiO, CuO and NiO−CuO is found to be 141.6, 42.6 and 92.1 nm respectively.

Raman Spectroscopy
The Raman spectra of NiO, CuO, and NiO−CuO thin films are shown in Figure 6a. Raman spectra of the pure NiO thin film reveal two broad bands at 520 cm −1 and 1064 cm −1 which are ascribed to NiO scattering of a first-order phonon (1P) and second-order phonon (2P), respectively [43]. CuO shows two Raman active modes at 270 cm −1 and 603 cm −1 , allocated as the A g and B g modes [44]. Additionally, NiO−CuO shows distinct characteristic bands of both NiO and CuO, confirming the presence of both metal oxides as shown in the XRD results. Figure 6b shows the Raman spectra of NiO, Cr 2 O 3, and Ni 0.95 Cr 0.05 O 2+δ . Three Raman bands of Cr 2 O 3 are observed at 300 cm −1 , 342 cm −1 , and 534 cm −1 assigned as E g , E g, and A g vibration modes [45]. In the Ni 0.95 Cr 0.05 O 2+δ spectrum, only vibrational modes of NiO appear. The sharpening of all of the Raman peaks of NiO in the Ni 0.95 Cr 0.05 O 2+δ spectrum suggests that doping results in increased crystallinity of the films, which correlates with the XRD results. ×120,000 magnification (e) Cr2O3, (f) Ni0.95Cr0.05O2+δ at magnification of ×60,000 and (g) Ni0.95Cr0.05O2+δ at ×120,000 magnification.

Raman Spectroscopy
The Raman spectra of NiO, CuO, and NiO−CuO thin films are shown in Figure 6a. Raman spectra of the pure NiO thin film reveal two broad bands at 520 cm −1 and 1064 cm −1 which are ascribed to NiO scattering of a first-order phonon (1P) and second-order phonon (2P), respectively [43]. CuO shows two Raman active modes at 270 cm −1 and 603 cm −1 , allocated as the Ag and Bg modes [44]. Additionally, NiO−CuO shows distinct characteristic bands of both NiO and CuO, confirming the presence of both metal oxides as shown in the XRD results. Figure 6b shows the Raman spectra of NiO, Cr2O3, and Ni0.95Cr0.05O2+δ. Three Raman bands of Cr2O3 are observed at 300 cm −1 , 342 cm −1 , and 534 cm −1 assigned as Eg, Eg, and Ag vibration modes [45]. In the Ni0.95Cr0.05O2+δ spectrum, only vibrational modes of NiO appear. The sharpening of all of the Raman peaks of NiO in the Ni0.95Cr0.05O2+δ spectrum suggests that doping results in increased crystallinity of the films, which correlates with the XRD results.

Surface Roughness Measurements
The AFM results of the NiO, CuO, NiO−CuO, Cr2O3, and Ni0.95Cr0.05O2+δ thin films are shown in Figure 7a-e. The root-mean-square-roughness (RMS) values of NiO, CuO, NiO−CuO, Cr2O3, and Ni0.95Cr0.05O2+δ thin films are found to be 17.3, 40.2, 257, 3, and 12.4 nm, respectively. These results reveal that the surface roughness of all the films are high, however, the roughness of the NiO−CuO and Ni0.95Cr0.05O2+δ thin films are higher compared to the NiO, CuO, and Cr2O3 thin films. It is believed that high surface roughness facilitates deeper electrode-electrolyte interaction and enhances the active surface area for the catalytic oxidation of methanol.

Surface Roughness Measurements
The The Raman spectra of NiO, CuO, and NiO−CuO thin films are shown in Figure 6a. Raman spectra of the pure NiO thin film reveal two broad bands at 520 cm −1 and 1064 cm −1 which are ascribed to NiO scattering of a first-order phonon (1P) and second-order phonon (2P), respectively [43]. CuO shows two Raman active modes at 270 cm −1 and 603 cm −1 , allocated as the Ag and Bg modes [44]. Additionally, NiO−CuO shows distinct characteristic bands of both NiO and CuO, confirming the presence of both metal oxides as shown in the XRD results. Figure 6b shows the Raman spectra of NiO, Cr2O3, and Ni0.95Cr0.05O2+δ. Three Raman bands of Cr2O3 are observed at 300 cm −1 , 342 cm −1 , and 534 cm −1 assigned as Eg, Eg, and Ag vibration modes [45]. In the Ni0.95Cr0.05O2+δ spectrum, only vibrational modes of NiO appear. The sharpening of all of the Raman peaks of NiO in the Ni0.95Cr0.05O2+δ spectrum suggests that doping results in increased crystallinity of the films, which correlates with the XRD results.

Surface Roughness Measurements
The AFM results of the NiO, CuO, NiO−CuO, Cr2O3, and Ni0.95Cr0.05O2+δ thin films are shown in Figure 7a-e. The root-mean-square-roughness (RMS) values of NiO, CuO, NiO−CuO, Cr2O3, and Ni0.95Cr0.05O2+δ thin films are found to be 17.3, 40.2, 257, 3, and 12.4 nm, respectively. These results reveal that the surface roughness of all the films are high, however, the roughness of the NiO−CuO and Ni0.95Cr0.05O2+δ thin films are higher compared to the NiO, CuO, and Cr2O3 thin films. It is believed that high surface roughness facilitates deeper electrode-electrolyte interaction and enhances the active surface area for the catalytic oxidation of methanol.

Electrochemical Oxidation of Methanol
The electrooxidation of methanol by NiO−CuO and Ni 0.95 Cr 0.05 O 2+δ thin films were investigated in 0.5 M NaOH, with different concentrations of methanol, ranging from 0 M to 0.3 M at a scan rate of 50 mVs −1 . It is believed that the anodic current density and the onset potential are the major factors in determining a catalyst's electrochemical oxidation potential. The LSV curves of the NiO, CuO, and NiO−CuO thin films in Figure 8a-c demonstrate that the current density of catalysts continually increases during methanol oxidation as the concentration of methanol increases, for up to 0.3 M. Moreover, the onset potential of catalysts for methanol electro-oxidation appeared at 0.43 V for the addition of methanol. Figure 8d reveals that all the tested catalysts exhibit catalytic activity for methanol oxidation, but the electrocatalytic activity of NiO−CuO thin film is higher than of pure NiO and pure CuO thin film. The current densities of NiO, CuO, and NiO−CuO thin films are 4.2 mA·cm −2 , 2 mA/0.5 cm −2 , and 6.1 mA/0.5 cm −2 (J = 12.2 mA·cm −2 ) at 0.6 V vs Ag/AgCl in the presence of 0.3 M methanol, respectively. The enhancement of methanol oxidation for the NiO−CuO catalyst can be attributed to the synergistic role of both metal oxides. This synergistic effect is observed when both metal oxides are mixed due to electronic interactions. The highest current density and lowest onset potential value were found in NiO−CuO, due to the presence of the electro-active species alpha-Ni(OH) 2 and the higher absorption ability of CuO, indicating that the splitting and oxidation of methanol molecules is easier at a lower potential.    Electrochemical impedance spectroscopy (EIS) is useful for determining the parameters that influence an electrode's efficiency, such as its charge transfer and diffusion properties. Figure 10a,b displays the Nyquist plot of NiO−CuO and Ni0.95Cr0.05O2+δ thin-film measured at the potential of 0.6 V in a frequency range of 100 KHz to 1 Hz in 0.5 M NaOH before and after the addition of 0.3 M methanol. The Nyquist plot displays two semicircles, one in the high-frequency region related to solution resistance and a second in the low-frequency region related to charging transfer resistance (Rct). Rct indicates the rate of charge exchange at the electrochemical interface between an aqueous solution and composite ions of the electrolyte. For the EIS plots of both NiO−CuO and Ni0.95Cr0.05O2+δ thin films, a smaller diameter of the second semicircle can be observed after the addition of methanol, which suggests fast electron-charge transfer. After the addition of methanol, the value of Rct calculated for NiO−CuO and Ni0.95Cr0.05O2+δ thin film is 3.18 Ω/cm 2 and 2.85 Ω/cm 2 respectively. These small Rct values in methanol indicate low charge transfer resistance, improved conductivity, and a higher catalytic activity for NiO−CuO and Ni0.95Cr0.05O2+δ electrocatalysts. Electrochemical impedance spectroscopy (EIS) is useful for determining the parameters that influence an electrode's efficiency, such as its charge transfer and diffusion properties. Figure 10a,b displays the Nyquist plot of NiO−CuO and Ni 0.95 Cr 0.05 O 2+δ thinfilm measured at the potential of 0.6 V in a frequency range of 100 KHz to 1 Hz in 0.5 M NaOH before and after the addition of 0.3 M methanol. The Nyquist plot displays two semicircles, one in the high-frequency region related to solution resistance and a second in the low-frequency region related to charging transfer resistance (R ct ). R ct indicates the rate of charge exchange at the electrochemical interface between an aqueous solution and composite ions of the electrolyte. For the EIS plots of both NiO−CuO and Ni 0.95 Cr 0.05 O 2+δ thin films, a smaller diameter of the second semicircle can be observed after the addition of methanol, which suggests fast electron-charge transfer. After the addition of methanol, the value of R ct calculated for NiO−CuO and Ni 0.95 Cr 0.05 O 2+δ thin film is 3.18 Ω/cm 2 and 2.85 Ω/cm 2 respectively. These small R ct values in methanol indicate low charge transfer resistance, improved conductivity, and a higher catalytic activity for NiO−CuO and Ni 0.95 Cr 0.05 O 2+δ electrocatalysts. A chronoamperometric test was carried out to characterize the stability of the methanol oxidation reaction for the NiO−CuO (Figure 11a) and Ni0.95Cr0.05O2+δ (Figure 11b) thin films in 0.5 M NaOH and 0.3 M methanol. Figure 11a,b shows that both samples displayed some current decay in the first few seconds, after which a relatively steady state was reached. This decay may be due to the adsorption of reaction intermediates such as CO. Furthermore, this decrease in current density may be due to a decrease in the methanol concentration near the electrode surface due to the rapid oxidation of methanol at the start. Subsequently, these changes occur under the mass transfer control process. NiO−CuO and Ni0.95Cr0.05O2+δ thin films have shown a stability of 95% and 89%, respectively, at a potential of 0.6 V for 2000 s. Moreover, NiO−CuO and Ni0.95Cr0.05O2+δ electrodes have maintained catalytic activity and stability toward methanol oxidation over a long period.   Figure 11a,b shows that both samples displayed some current decay in the first few seconds, after which a relatively steady state was reached. This decay may be due to the adsorption of reaction intermediates such as CO. Furthermore, this decrease in current density may be due to a decrease in the methanol concentration near the electrode surface due to the rapid oxidation of methanol at the start. Subsequently, these changes occur under the mass transfer control process. NiO−CuO and Ni 0.95 Cr 0.05 O 2+δ thin films have shown a stability of 95% and 89%, respectively, at a potential of 0.6 V for 2000 s. Moreover, NiO−CuO and Ni 0.95 Cr 0.05 O 2+δ electrodes have maintained catalytic activity and stability toward methanol oxidation over a long period. A chronoamperometric test was carried out to characterize the stability of the methanol oxidation reaction for the NiO−CuO (Figure 11a) and Ni0.95Cr0.05O2+δ (Figure 11b) thin films in 0.5 M NaOH and 0.3 M methanol. Figure 11a,b shows that both samples displayed some current decay in the first few seconds, after which a relatively steady state was reached. This decay may be due to the adsorption of reaction intermediates such as CO. Furthermore, this decrease in current density may be due to a decrease in the methanol concentration near the electrode surface due to the rapid oxidation of methanol at the start. Subsequently, these changes occur under the mass transfer control process. NiO−CuO and Ni0.95Cr0.05O2+δ thin films have shown a stability of 95% and 89%, respectively, at a potential of 0.6 V for 2000 s. Moreover, NiO−CuO and Ni0.95Cr0.05O2+δ electrodes have maintained catalytic activity and stability toward methanol oxidation over a long period.

Conclusions
In summary, NiO−CuO and Ni 0.95 Cr 0.05 O 2+δ thin films were fabricated on FTO glass substrates via a facile dip-coating method followed by calcination at 500 • C. The films were appropriately characterized through different analytical techniques such XRD, XPS, FESEM, EDX. The uniformly distributed films on FTO glass substrate were then examined for their