Microwave-Assisted Synthesis of Co 3 ( PO 4 ) 2 Nanospheres for Electrocatalytic Oxidation of Methanol in Alkaline Media

Low-cost and high-performance advanced electrocatalysts for direct methanol fuel cells are of key significance for the improvement of environmentally-pleasant energy technologies. Herein, we report the facile synthesis of cobalt phosphate (Co3(PO4)2) nanospheres by a microwave-assisted process and utilized as an electrocatalyst for methanol oxidation. The phase formation, morphological surface structure, elemental composition, and textural properties of the synthesized (Co3(PO4)2) nanospheres have been examined by powder X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR), field emission-scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption-desorption isotherm investigations. The performance of an electrocatalytic oxidation of methanol over a Co3(PO4)2 nanosphere-modified electrode was evaluated in an alkaline solution using cyclic voltammetry (CV) and chronopotentiometry (CP) techniques. Detailed studies were made for the methanol oxidation by varying the experimental parameters, such as catalyst loading, methanol concentration, and long-term stability for the electro-oxidation of methanol. The good electrocatalytic performances of Co3(PO4)2 should be related to its good surface morphological structure and high number of active surface sites. The present investigation illustrates the promising application of Co3(PO4)2 nanospheres as a low-cost and more abundant electrocatalyst for direct methanol fuel cells.


Introduction
Concerns of the expanding demand for energy by humankind and the quick consumption of fossil fuels have set off the need to plan and create novel advances for the production of energy in a sustainable way [1,2].Direct alcohol fuel cells (DAFCs) have engrossed significant interest as a power source for handy applications, such as laptops, mobile phones, automobiles, and digital cameras, etc. [3].The concerns with storage and transport of alcohols (such as ethanol, methanol, glycerol, and ethylene glycol) are more user-responsive compared to hydrogen [4].The most frequent DAFC is the direct methanol fuel cell (DMFC), which relies upon the oxidation of methanol over the surface of the catalysts.In recent years, DMFCs are considered as one of the most promising candidate for solving the energy crisis because of their high energy conversion efficiency, non-toxic nature, less noise, and low operating temperature [5,6].Electrocatalytic oxidation of methanol is the prime step in the progress of DMFCs.Conventionally, platinum and platinum-based alloys are extensively used as promising electrocatalysts for the oxidation of alcohols due to their outstanding electrocatalytic activities.However, what strongly limit their large-scale applications are the expensive noble metals and their lower abundance in nature.This concern has brought more attention on researchers to develop an alternative low-cost and high catalytic performance for the electrocatalytic oxidation of alcohols.
Over the past few years, several other functional electrocatalytic materials have been engaged as alternatives to noble Pt electrocatalysts in DMFCs which consist of metal oxides [7,8], carbonaceous materials [9,10], metal chalcogenides [11][12][13], and phosphates [14,15].More recently, metal phosphates have exposed as an efficient electrocatalysts for the uses in various applications due to their exclusive physical and chemical properties.Many researchers have examined Co-based electrocatalysts for the oxidation of alcohols and energy-related applications [16][17][18][19][20][21][22][23][24].Due to the mesoporous-structured nature, highly active surface area, non-toxic nature, and greater abundance of Co-based electrocatalyst show an enhanced property for several applications.
Herein, we report a microwave-assisted process for the fabrication of porous Co 3 (PO 4 ) 2 nanospheres.In this work, we have used a microwave-assisted process in order to avoid particle aggregation during the precipitation of Co 3 (PO 4 ) 2 nanospheres and an arrangement of good morphological features.The good surface morphological features of the catalyst are always desired to improve the electrocatalytic performance.The electrocatalytic activity of the synthesized Co 3 (PO 4 ) 2 nanospheres was examined by the electrocatalytic methanol oxidation.The crystallinity, purity of phase, textural properties, and electrochemical performances of synthesized Co 3 (PO 4 ) 2 were examined by powder X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET), and electrochemical (voltammetric) measurements.

Powder XRD Analysis
The crystallinity and phase formation of the Co 3 (PO 4 ) 2 materials was examined by powder XRD analysis and the resulting XRD pattern is presented in Figure 1.The diffraction peaks of the calcined Co 3 (PO 4 ) 2 at 600 • C (Figure 1d) showed numerous peaks which are consistent with the diffraction planes.The detected (hkl) planes are indexed, agreeing with the stated hexagonal phase of Co 3 (PO 4 ) 2 (JCPDS No. 70-1795, Figure 1a).Conversely, it is observed that the XRD pattern of the as-synthesized (Figure 1b) and calcined Co 3 (PO 4 ) 2 at 300 • C (Figure 1c) showed two broad humps at a 2θ value of 35 • and 55 • (Figure 1c).This broad peak established that the Co 3 (PO 4 ) 2 showed an amorphous nature [19,20].The calcined Co 3 (PO 4 ) 2 at 600 • C sample shows the strong crystalline nature of Co 3 (PO 4 ) 2 compared to others.We have also measured the average crystalline size, which was observed to be 32.07 nm for calcined Co 3 (PO 4 ) 2 .
Catalysts 2017, 7, 119 2 of 11 extensively used as promising electrocatalysts for the oxidation of alcohols due to their outstanding electrocatalytic activities.However, what strongly limit their large-scale applications are the expensive noble metals and their lower abundance in nature.This concern has brought more attention on researchers to develop an alternative low-cost and high catalytic performance for the electrocatalytic oxidation of alcohols.
Over the past few years, several other functional electrocatalytic materials have been engaged as alternatives to noble Pt electrocatalysts in DMFCs which consist of metal oxides [7,8], carbonaceous materials [9,10], metal chalcogenides [11][12][13], and phosphates [14,15].More recently, metal phosphates have exposed as an efficient electrocatalysts for the uses in various applications due to their exclusive physical and chemical properties.Many researchers have examined Co-based electrocatalysts for the oxidation of alcohols and energy-related applications [16][17][18][19][20][21][22][23][24].Due to the mesoporous-structured nature, highly active surface area, non-toxic nature, and greater abundance of Co-based electrocatalyst show an enhanced property for several applications.
Herein, we report a microwave-assisted process for the fabrication of porous Co3(PO4)2 nanospheres.In this work, we have used a microwave-assisted process in order to avoid particle aggregation during the precipitation of Co3(PO4)2 nanospheres and an arrangement of good morphological features.The good surface morphological features of the catalyst are always desired to improve the electrocatalytic performance.The electrocatalytic activity of the synthesized Co3(PO4)2 nanospheres was examined by the electrocatalytic methanol oxidation.The crystallinity, purity of phase, textural properties, and electrochemical performances of synthesized Co3(PO4)2 were examined by powder X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET), and electrochemical (voltammetric) measurements.

Powder XRD Analysis
The crystallinity and phase formation of the Co3(PO4)2 materials was examined by powder XRD analysis and the resulting XRD pattern is presented in Figure 1.The diffraction peaks of the calcined Co3(PO4)2 at 600 °C (Figure 1d) showed numerous peaks which are consistent with the diffraction planes.The detected (hkl) planes are indexed, agreeing with the stated hexagonal phase of Co3(PO4)2 (JCPDS No. 70-1795, Figure 1a).Conversely, it is observed that the XRD pattern of the as-synthesized (Figure 1b) and calcined Co3(PO4)2 at 300 °C (Figure 1c) showed two broad humps at a 2θ value of 35° and 55° (Figure 1c).This broad peak established that the Co3(PO4)2 showed an amorphous nature [19,20].The calcined Co3(PO4)2 at 600 °C sample shows the strong crystalline nature of Co3(PO4)2 compared to others.We have also measured the average crystalline size, which was observed to be 32.07 nm for calcined Co3(PO4)2.

FT-IR Spectroscopy Analysis
The chemical interactions occurred in Co 3 (PO 4 ) 2 nanospheres were examined by FT-IR spectroscopy.The FT-IR spectra of Co 3 (PO 4 ) 2 was evident in the region of 500-4000 cm −1 and the experiential spectrum is shown in Figure 2. In the Co 3 (PO 4 ) 2 spectrum, the vibrational band at 689 cm −1 is ascribed to the V 4 (F 2 )PO 4 3− mode of the phosphate (PO 4 ) group [25].The broad characteristic peaks at 1017 and 1365 cm −1 are attributed to the survival of the P=O stretching vibration in PO 4 3− and the P-O-P stretching vibration in the HPO 4 2− group, respectively [26].The distinguished broad vibration peak at 3425 cm −1 is assigned to the stretching frequency of the O-H group.The FT-IR results distinctly suggested the subsistence of the Co 3 (PO 4 ) 2 sample.

FTIR Spectroscopy Analysis
The chemical interactions occurred in Co3(PO4)2 nanospheres were examined by FTIR spectroscopy.The FTIR spectra of Co3(PO4)2 was evident in the region of 500-4000 cm −1 and the experiential spectrum is shown in Figure 2. In the Co3(PO4)2 spectrum, the vibrational band at 689 cm −1 is ascribed to the V4(F2)PO4 3− mode of the phosphate (PO4) group [25].The broad characteristic peaks at 1017 and 1365 cm −1 are attributed to the survival of the P=O stretching vibration in PO4 3− and the P-O-P stretching vibration in the HPO4 2− group, respectively [26].The distinguished broad vibration peak at 3425 cm −1 is assigned to the stretching frequency of the O-H group.The FTIR results distinctly suggested the subsistence of the Co3(PO4)2 sample.

Morphological Analysis
The surface morphology of the porous Co3(PO4)2 materials were analyzed by FE-SEM and the resultant images are displayed in Figure 3a,b.FESEM results of Co3(PO4)2 show nanosphere like morphology and form a hierarchical nanostructure with uniformly-packed Co3(PO4)2 with an average size of ~100 to 200 nm.It can be noticed that the surface morphology of the Co3(PO4)2 sample are tightly-packed nanosphere-like particles which can offer support for effective electron transport [27].Further in-depth exploration on the surface morphology of Co3(PO4)2 samples was observed by HRTEM and the equivalent images are shown in Figure 4a,b.The HRTEM images also confirmed the nanosphere-like surface morphology of Co3(PO4)2 samples having an average particle size of ~100 nm.

Morphological Analysis
The surface morphology of the porous Co 3 (PO 4 ) 2 materials were analyzed by FE-SEM and the resultant images are displayed in Figure 3a,b.FE-SEM results of Co 3 (PO 4 ) 2 show nanosphere like morphology and form a hierarchical nanostructure with uniformly-packed Co 3 (PO 4 ) 2 with an average size of ~100 to 200 nm.It can be noticed that the surface morphology of the Co 3 (PO 4 ) 2 sample are tightly-packed nanosphere-like particles which can offer support for effective electron transport [27].Further in-depth exploration on the surface morphology of Co 3 (PO 4 ) 2 samples was observed by HRTEM and the equivalent images are shown in Figure 4a,b.The HRTEM images also confirmed the nanosphere-like surface morphology of Co 3 (PO 4 ) 2 samples having an average particle size of ~100 nm.

FTIR Spectroscopy Analysis
The chemical interactions occurred in Co3(PO4)2 nanospheres were examined by FTIR spectroscopy.The FTIR spectra of Co3(PO4)2 was evident in the region of 500-4000 cm −1 and the experiential spectrum is shown in Figure 2. In the Co3(PO4)2 spectrum, the vibrational band at 689 cm −1 is ascribed to the V4(F2)PO4 3− mode of the phosphate (PO4) group [25].The broad characteristic peaks at 1017 and 1365 cm −1 are attributed to the survival of the P=O stretching vibration in PO4 3− and the P-O-P stretching vibration in the HPO4 2− group, respectively [26].The distinguished broad vibration peak at 3425 cm −1 is assigned to the stretching frequency of the O-H group.The FTIR results distinctly suggested the subsistence of the Co3(PO4)2 sample.

Morphological Analysis
The surface morphology of the porous Co3(PO4)2 materials were analyzed by FE-SEM and the resultant images are displayed in Figure 3a,b.FESEM results of Co3(PO4)2 show nanosphere like morphology and form a hierarchical nanostructure with uniformly-packed Co3(PO4)2 with an average size of ~100 to 200 nm.It can be noticed that the surface morphology of the Co3(PO4)2 sample are tightly-packed nanosphere-like particles which can offer support for effective electron transport [27].Further in-depth exploration on the surface morphology of Co3(PO4)2 samples was observed by HRTEM and the equivalent images are shown in Figure 4a,b.The HRTEM images also confirmed the nanosphere-like surface morphology of Co3(PO4)2 samples having an average particle size of ~100 nm.

XPS Analysis
The elemental compositions and chemical bonding of the synthesized Co3(PO4)2 nanospheres was investigated by XPS.The survey spectra of Co3(PO4)2 (Figure 5a) clearly evidenced the presence of Co, P, and O elements in the synthesized Co3(PO4)2 sample.The high-resolution XPS spectra for Co2p, P2p, and O1s species are presented in Figure 5b-d.The binding energy of Co2p (Figure 5b) appeared in two peaks around 782 and 798 eV, which are ascribed to Co2p3/2 and Co2p1/2, respectively and also with two shake-up satellites.Figure 5c demonstrated the binding energy of P2p which is centered at 134 eV.The peaks centered at 133.4, 134.1, and 135.2 eV related to the characteristics peaks of P(V) of the Co3(PO4)2 sample (Figure 5c).The observed binding energy around 532 eV of O1s is attributed to the bonding of metal oxide (Figure 5d).The fitted peak at 531.6 can related to the metal-oxygen bonds and the peak at 532.6 eV related to the oxygen of OH groups.Further, the peak at 533.9 can be related to the adsorbed water near the surface.(Figure 5d).The examined XPS analysis clearly demonstrated the formation of Co3(PO4)2 materials and the obtained results are consistent with powder XRD analysis.

XPS Analysis
The elemental compositions and chemical bonding of the synthesized Co 3 (PO 4 ) 2 nanospheres was investigated by XPS.The survey spectra of Co 3 (PO 4 ) 2 (Figure 5a) clearly evidenced the presence of Co, P, and O elements in the synthesized Co 3 (PO 4 ) 2 sample.The high-resolution XPS spectra for Co 2p , P 2p , and O 1s species are presented in Figure 5b-d.The binding energy of Co 2p (Figure 5b) appeared in two peaks around 782 and 798 eV, which are ascribed to Co 2p3/2 and Co 2p1/2 , respectively and also with two shake-up satellites.Figure 5c demonstrated the binding energy of P 2p which is centered at 134 eV.The peaks centered at 133.4, 134.1, and 135.2 eV related to the characteristics peaks of P(V) of the Co 3 (PO 4 ) 2 sample (Figure 5c).The observed binding energy around 532 eV of O1s is attributed to the bonding of metal oxide (Figure 5d).The fitted peak at 531.6 can related to the metal-oxygen bonds and the peak at 532.6 eV related to the oxygen of OH groups.Further, the peak at 533.9 can be related to the adsorbed water near the surface.(Figure 5d).The examined XPS analysis clearly demonstrated the formation of Co 3 (PO 4 ) 2 materials and the obtained results are consistent with powder XRD analysis.

XPS Analysis
The elemental compositions and chemical bonding of the synthesized Co3(PO4)2 nanospheres was investigated by XPS.The survey spectra of Co3(PO4)2 (Figure 5a) clearly evidenced the presence of Co, P, and O elements in the synthesized Co3(PO4)2 sample.The high-resolution XPS spectra for Co2p, P2p, and O1s species are presented in Figure 5b-d.The binding energy of Co2p (Figure 5b) appeared in two peaks around 782 and 798 eV, which are ascribed to Co2p3/2 and Co2p1/2, respectively and also with two shake-up satellites.Figure 5c demonstrated the binding energy of P2p which is centered at 134 eV.The peaks centered at 133.4, 134.1, and 135.2 eV related to the characteristics peaks of P(V) of the Co3(PO4)2 sample (Figure 5c).The observed binding energy around 532 eV of O1s is attributed to the bonding of metal oxide (Figure 5d).The fitted peak at 531.6 can related to the metal-oxygen bonds and the peak at 532.6 eV related to the oxygen of OH groups.Further, the peak at 533.9 can be related to the adsorbed water near the surface.(Figure 5d).The examined XPS analysis clearly demonstrated the formation of Co3(PO4)2 materials and the obtained results are consistent with powder XRD analysis.

Textural Properties
The nitrogen adsorption-desorption isotherm analysis was carried out to examine the textural properties, such as BET surface area, pore size, and pore volume of the synthesized Co 3 (PO 4 ) 2 nanospheres.The adsorption-desorption isotherms of Co 3 (PO 4 ) 2 nanospheres are displayed in Figure 6.Based on IUPAC classification, the isotherm of Co 3 (PO 4 ) 2 nanospheres exposes a type IV isotherm with distinct H3 hysteresis loops in a P/P 0 range of 0.1 to 1, which is related to the porous materials.The examined specific BET surface area value from adsorption-desorption isotherms was 20.2 m 2 /g.In addition, Figure 6 (inset) demonstrates the BJH pore size distribution of Co 3 (PO 4 ) 2 nanospheres with average pore size and volume around 0.98 nm and 1.001 × 10 −2 mL/g, respectively.It can be noted that the good surface morphology and textural properties are benefits for the enhancement of electrode/electrolyte interfacial area, which can improve the electrocatalytic active sites for electrocatalytic oxidation of methanol.

Textural Properties
The nitrogen adsorption-desorption isotherm analysis was carried out to examine the textural properties, such as BET surface area, pore size, and pore volume of the synthesized Co3(PO4)2 nanospheres.The adsorption-desorption isotherms of Co3(PO4)2 nanospheres are displayed in Figure 6.Based on IUPAC classification, the isotherm of Co3(PO4)2 nanospheres exposes a type IV isotherm with distinct H3 hysteresis loops in a P/P0 range of 0.1 to 1, which is related to the porous materials.The examined specific BET surface area value from adsorption-desorption isotherms was 20.2 m 2 /g.In addition, Figure 6 (inset) demonstrates the BJH pore size distribution of Co3(PO4)2 nanospheres with average pore size and volume around 0.98 nm and 1.001 × 10 −2 mL/g, respectively.
It can be noted that the good surface morphology and textural properties are benefits for the enhancement of electrode/electrolyte interfacial area, which can improve the electrocatalytic active sites for electrocatalytic oxidation of methanol.6 shows the pore size distribution.

Electrocatalytic Methanol Oxidation over Co3(PO4)2 Nanospheres
The electrochemical performances of the Co3(PO4)2 nanosphere catalyst, with regard to oxidation-reduction reactions in alkaline media, were evaluated and compared with bulk Co3(PO4)2 fabricated in the absence of any reducing agent.Figure 7a displays the cyclic voltammetry at 50 mV•s −1 in 1.0 M KOH solution for loaded catalysts on an Au electrode.The cyclic voltammograms (CV) show two redox peaks (A1/C1 = Co 2+ /Co 3+ ) and (A2/C2 = Co 3+ /Co 4+ ) located at about 1.12/1.08 and 1.45/1.38V vs. RHE, respectively, for Co3(PO4)2 nanospheres and are typically similar to those reported in the literature [28].The CV curves of Co3(PO4)2-modified electrode (Figure 7a) exhibit a redox peak prior to the oxygen evolution reactions can be credited to the well-known redox reaction of Co(II)/Co(III) and a feasible electrochemical reaction in a KOH solution is afforded in reaction (1): It is evidenced that the redox peak currents of Co3(PO4)2 nanosphere catalyst voltammograms are significantly higher (15 times) than the current for bulk Co3(PO4)2 signifying substantially higher electrochemical area and higher capacitance current for Co3(PO4)2 nanosphere catalysts.
The profile of CVs of the Co3(PO4)2-modified electrode at a scan speed of 50 mV•s −1 in 1.0 M KOH solution with and without the presence of 1.0 M methanol is exhibited in Figure 7b.Prior to examining the electrocatalytic methanol oxidation activity of Co3(PO4)2 nanospheres, the electrochemical properties of the Co3(PO4)2 nanosphere-modified electrode was scrutinized in 1.0 M KOH solution.The CV of the Co3(PO4)2-modified electrode (Figure 7b) in 1.0 M KOH + 1.0 M CH3OH shows a considerable increment in the oxidation peak current density at an oxidation peak potential of 1.65 V (vs.RHE) and also noticed that the decrease in the redox peak during the reverse scan.In Figure 6.N 2 adsorption-desorption isotherms of Co 3 (PO 4 ) 2 nanospheres.The inset of Figure 6 shows the pore size distribution.

Electrocatalytic Methanol Oxidation over Co 3 (PO 4 ) 2 Nanospheres
The electrochemical performances of the Co 3 (PO 4 ) 2 nanosphere catalyst, with regard to oxidation-reduction reactions in alkaline media, were evaluated and compared with bulk Co 3 (PO 4 ) 2 fabricated in the absence of any reducing agent.Figure 7a displays the cyclic voltammetry at 50 mV•s −1 in 1.0 M KOH solution for loaded catalysts on an Au electrode.The cyclic voltammograms (CV) show two redox peaks (A 1 /C 1 = Co 2+ /Co 3+ ) and (A 2 /C 2 = Co 3+ /Co 4+ ) located at about 1.12/1.08 and 1.45/1.38V vs. RHE, respectively, for Co 3 (PO 4 ) 2 nanospheres and are typically similar to those reported in the literature [28].The CV curves of Co 3 (PO 4 ) 2 -modified electrode (Figure 7a) exhibit a redox peak prior to the oxygen evolution reactions can be credited to the well-known redox reaction of Co(II)/Co(III) and a feasible electrochemical reaction in a KOH solution is afforded in reaction (1): It is evidenced that the redox peak currents of Co 3 (PO 4 ) 2 nanosphere catalyst voltammograms are significantly higher (15 times) than the current for bulk Co 3 (PO 4 ) 2 signifying substantially higher electrochemical area and higher capacitance current for Co 3 (PO 4 ) 2 nanosphere catalysts.
The profile of CVs of the Co 3 (PO 4 ) 2 -modified electrode at a scan speed of 50 mV•s −1 in 1.0 M KOH solution with and without the presence of 1.0 M methanol is exhibited in Figure 7b.Prior to examining the electrocatalytic methanol oxidation activity of Co 3 (PO 4 ) 2 nanospheres, the electrochemical properties of the Co 3 (PO 4 ) 2 nanosphere-modified electrode was scrutinized in 1.0 M KOH solution.The CV of the Co 3 (PO 4 ) 2 -modified electrode (Figure 7b) in 1.0 M KOH + 1.0 M CH 3 OH shows a considerable increment in the oxidation peak current density at an oxidation peak potential of 1.65 V (vs.RHE) and also noticed that the decrease in the redox peak during the reverse scan.In addition, the methanol catalytic oxidation current tends to increase with the positive scan until reaching a plateau of about 120 mA/cm 2 at 1.7 V vs. RHE.Moreover, it is observed that the oxygen evolution reaction occurs at more positive potentials in the presence of methanol solutions which can be clarified by the greater affinity of Co 3+ intermediate species (CoOOH) to adsorb methanol than to OH − ions [29].Thus, the increased oxidation current and the departed redox peak in the presence of CH 3 OH in KOH, when compared to pure KOH solution, revealed the electrocatalytic activity of Co 3 (PO 4 ) 2 for the oxidation of methanol.The electrocatalytic performances of Co 3 (PO 4 ) 2 are nearly related to its good surface morphology and high active surface area, which was completely investigated with the results obtained above in FE-SEM, HRTEM, and BET analysis.addition, the methanol catalytic oxidation current tends to increase with the positive scan until reaching a plateau of about 120 mA/cm 2 at 1.7 V vs. RHE.Moreover, it is observed that the oxygen evolution reaction occurs at more positive potentials in the presence of methanol solutions which can be clarified by the greater affinity of Co 3+ intermediate species (CoOOH) to adsorb methanol than to OH − ions [29].Thus, the increased oxidation current and the departed redox peak in the presence of CH3OH in KOH, when compared to pure KOH solution, revealed the electrocatalytic activity of Co3(PO4)2 for the oxidation of methanol.The electrocatalytic performances of Co3(PO4)2 are nearly related to its good surface morphology and high active surface area, which was completely investigated with the results obtained above in FESEM, HRTEM, and BET analysis.We have examined the effect of the catalyst loading amount on the electrocatalytic oxidation of methanol in 1.0 M KOH + 1.0 M CH3OH and the corresponding CV profile is shown in Figure 8a.It can be seen that the methanol oxidation current is noticeably enhanced.Figure 8b shows the plot of anodic methanol oxidation current density vs. electrocatalyst loading amount at 1.65 V vs. RHE, and this clearly reveals that the linear increase of the oxidation current with a catalyst loading amount (0.07 to 0.28 mg/cm 2 ) may be due to an increase in the number of accessible active catalyst sites and efficient mass transport ions into the Co3(PO4)2 catalyst [29].Additionally, it can be seen that the oxidation potential is shifted towards the negative potential when the electrocatalyst loading amount increased, which suggests good kinetic performance for the oxidation of methanol.We have examined the effect of the catalyst loading amount on the electrocatalytic oxidation of methanol in 1.0 M KOH + 1.0 M CH 3 OH and the corresponding CV profile is shown in Figure 8a.It can be seen that the methanol oxidation current is noticeably enhanced.Figure 8b shows the plot of anodic methanol oxidation current density vs. electrocatalyst loading amount at 1.65 V vs. RHE, and this clearly reveals that the linear increase of the oxidation current with a catalyst loading amount (0.07 to 0.28 mg/cm 2 ) may be due to an increase in the number of accessible active catalyst sites and efficient mass transport ions into the Co 3 (PO 4 ) 2 catalyst [29].Additionally, it can be seen that the oxidation potential is shifted towards the negative potential when the electrocatalyst loading amount increased, which suggests good kinetic performance for the oxidation of methanol.
Catalysts 2017, 7, 119 6 of 11 addition, the methanol catalytic oxidation current tends to increase with the positive scan until reaching a plateau of about 120 mA/cm 2 at 1.7 V vs. RHE.Moreover, it is observed that the oxygen evolution reaction occurs at more positive potentials in the presence of methanol solutions which can be clarified by the greater affinity of Co 3+ intermediate species (CoOOH) to adsorb methanol than to OH − ions [29].Thus, the increased oxidation current and the departed redox peak in the presence of CH3OH in KOH, when compared to pure KOH solution, revealed the electrocatalytic activity of Co3(PO4)2 for the oxidation of methanol.The electrocatalytic performances of Co3(PO4)2 are nearly related to its good surface morphology and high active surface area, which was completely investigated with the results obtained above in FESEM, HRTEM, and BET analysis.We have examined the effect of the catalyst loading amount on the electrocatalytic oxidation of methanol in 1.0 M KOH + 1.0 M CH3OH and the corresponding CV profile is shown in Figure 8a.It can be seen that the methanol oxidation current is noticeably enhanced.Figure 8b shows the plot of anodic methanol oxidation current density vs. electrocatalyst loading amount at 1.65 V vs. RHE, and this clearly reveals that the linear increase of the oxidation current with a catalyst loading amount (0.07 to 0.28 mg/cm 2 ) may be due to an increase in the number of accessible active catalyst sites and efficient mass transport ions into the Co3(PO4)2 catalyst [29].Additionally, it can be seen that the oxidation potential is shifted towards the negative potential when the electrocatalyst loading amount increased, which suggests good kinetic performance for the oxidation of methanol.The CVs at 50 mV/s for 0.28 mg/cm 2 loaded Co 3 (PO 4 ) 2 nanospheres in various concentrations of CH 3 OH solution are presented in Figure 9a.It can be observed that the CH 3 OH oxidation peak current density gradually increases with a shifting of the peak potential towards the positive potential as the concentration of methanol increases (Figure 9a).The linear relationship between the methanol concentration and the oxidation peak current at 1.65 V vs. RHE is plotted and shown in Figure 9b.This clearly showed the increase of methanol oxidation current linearly with the concentration of the methanol solution, which illustrates that Co 3 (PO 4 ) 2 nanospheres have good tolerance for product adsorption [29].
Catalysts 2017, 7, 119 7 of 11 The CVs at 50 mV/s for 0.28 mg/cm 2 loaded Co3(PO4)2 nanospheres in various concentrations of CH3OH solution are presented in Figure 9a.It can be observed that the CH3OH oxidation peak current density gradually increases with a shifting of the peak potential towards the positive potential as the concentration of methanol increases (Figure 9a).The linear relationship between the methanol concentration and the oxidation peak current at 1.65 V vs. RHE is plotted and shown in Figure 9b.This clearly showed the increase of methanol oxidation current linearly with the concentration of the methanol solution, which illustrates that Co3(PO4)2 nanospheres have good tolerance for product adsorption [29].The electrochemical long-term stability of the Co3(PO4)2 nanosphere-modified electrode is also an important factor for the electro-oxidation of methanol.The long-term stability was examined using chronopotentiometry measurements at two different current densities (25 and 75 mA/cm 2 ) in 0.14 mg/cm 2 loaded Co3(PO4)2 nanospheres and the obtained result is shown in Figure 10.It can be seen from Figure 10 that the Co3(PO4)2 nanosphere-modified electrode demonstrated nearly constant operating potentials at 1.53 and 1.73 V vs. RHE for 25 and 75 mA/cm 2 during 120 min of methanol oxidation testing.This suggests that Co3(PO4)2 nanospheres are relatively stable and tolerant of high poisoning levels for the oxidation of methanol.At the start of the reaction, the operating potentials are shown gradually increasing and reach steady-state which indicates incompletely-adsorbed oxidation products for a few minutes, initially.Table 1 displays the specific mass activities of nickeland cobalt-based catalyst for methanol electro-oxidation, and it is evidenced that the methanol electro-oxidation catalytic steady-state specific activities of the Co3(PO4)2 nanosphere catalyst is superior than other nickel-and cobalt-based catalysts.The electrostability of the Co3(PO4)2 electrocatalyst also confirmed the feasible application of Co3(PO4)2 nanospheres as a capable low-cost substitute to Pt-based electrocatalysts for methanol oxidation.Lastly, and in comparison with other nickel-and cobalt-based catalysts, it is found that the methanol oxidation is significantly higher than that for cobalt-based catalysts.However, detailed study needs to be carried out to establish the Co3(PO4)2 nanosphere-based electrocatalyst to be more competitive for the methanol electro-oxidation reaction.The electrochemical long-term stability of the Co 3 (PO 4 ) 2 nanosphere-modified electrode is also an important factor for the electro-oxidation of methanol.The long-term stability was examined using chronopotentiometry measurements at two different current densities (25 and 75 mA/cm 2 ) in 0.14 mg/cm 2 loaded Co 3 (PO 4 ) 2 nanospheres and the obtained result is shown in Figure 10.It can be seen from Figure 10 that the Co 3 (PO 4 ) 2 nanosphere-modified electrode demonstrated nearly constant operating potentials at 1.53 and 1.73 V vs. RHE for 25 and 75 mA/cm 2 during 120 min of methanol oxidation testing.This suggests that Co 3 (PO 4 ) 2 nanospheres are relatively stable and tolerant of high poisoning levels for the oxidation of methanol.At the start of the reaction, the operating potentials are shown gradually increasing and reach steady-state which indicates incompletely-adsorbed oxidation products for a few minutes, initially.Table 1 displays the specific mass activities of nickel-and cobalt-based catalyst for methanol electro-oxidation, and it is evidenced that the methanol electro-oxidation catalytic steady-state specific activities of the Co 3 (PO 4 ) 2 nanosphere catalyst is superior than other nickel-and cobalt-based catalysts.The electrostability of the Co 3 (PO 4 ) 2 electrocatalyst also confirmed the feasible application of Co 3 (PO 4 ) 2 nanospheres as a capable low-cost substitute to Pt-based electrocatalysts for methanol oxidation.Lastly, and in comparison with other nickel-and cobalt-based catalysts, it is found that the methanol oxidation is significantly higher than that for cobalt-based catalysts.However, detailed study needs to be carried out to establish the Co 3 (PO 4 ) 2 nanosphere-based electrocatalyst to be more competitive for the methanol electro-oxidation reaction.

Microwave-Assisted Synthesis of Co3(PO4)2 Nanospheres
The porous Co3(PO4)2 nanospheres were synthesized through a microwave-assisted process using cobalt nitrate (Co(NO3)2•6H2O; Sigma Aldrich, St. Louis, MO, USA) and di-sodium hydrogen phosphate (Na2HPO4; Sigma Aldrich) as the cobalt and phosphate sources, respectively.For instance, the evaluated amount of Co(NO3)2•6H2O (0.2 M) and Na2HPO4 (0.2 M) was dissolved along with 0.8 M of urea (NH2CONH2; Sigma Aldrich) in 40 mL of deionized water under ultrasonication for 30 min.Concisely, the amount of cobalt phosphate precursor and urea with a molar ratio of 1:4 were dissolved in 40 mL of water to obtain a homogenous solution.Then, 90 mL of ethylene glycol was added under ultrasonic agitation until the homogenous reaction mixture was obtained.The acquired homogenous solution was placed in a microwave oven and the microwave reaction was carried out under 600 W microwave irradiation for 15 min to form Co3(PO4)2 products.The obtained precipitate after filtration was processed via several washings with deionized water, absolute ethanol, and dried at 80 °C for 3 h.Finally, the obtained Co3(PO4)2 product was annealed for 1 h at 300 and 600 °C.For comparison purposes, the bulk Co3(PO4)2 particles were obtained and prepared without any reducing agent.

Microwave-Assisted Synthesis of Co 3 (PO 4 ) 2 Nanospheres
The porous Co 3 (PO 4 ) 2 nanospheres were synthesized through a microwave-assisted process using cobalt nitrate (Co(NO 3 ) 2 •6H 2 O; Sigma Aldrich, St. Louis, MO, USA) and di-sodium hydrogen phosphate (Na 2 HPO 4 ; Sigma Aldrich) as the cobalt and phosphate sources, respectively.For instance, the evaluated amount of Co(NO 3 ) 2 •6H 2 O (0.2 M) and Na 2 HPO 4 (0.2 M) was dissolved along with 0.8 M of urea (NH 2 CONH 2 ; Sigma Aldrich) in 40 mL of deionized water under ultrasonication for 30 min.Concisely, the amount of cobalt phosphate precursor and urea with a molar ratio of 1:4 were dissolved in 40 mL of water to obtain a homogenous solution.Then, 90 mL of ethylene glycol was added under ultrasonic agitation until the homogenous reaction mixture was obtained.The acquired homogenous solution was placed in a microwave oven and the microwave reaction was carried out under 600 W microwave irradiation for 15 min to form Co 3 (PO 4 ) 2 products.The obtained precipitate after filtration was processed via several washings with deionized water, absolute ethanol, and dried at 80 • C for 3 h.Finally, the obtained Co 3 (PO 4 ) 2 product was annealed for 1 h at 300 and 600 • C. For comparison purposes, the bulk Co 3 (PO 4 ) 2 particles were obtained and prepared without any reducing agent.

Instrumental Characterization
The crystallinity and phase formation of the synthesized Co 3 (PO 4 ) 2 sample was analyzed by powder XRD (Rigaku Miniflex 600, Rigaku corporation, Tokyo, Japan) with copper Kα radiation (λ = 1.5418Å) at a scan speed of 3 • /min.The chemical interactions that occurred in Co 3 (PO 4 ) 2 were examined using FT-IR spectroscopy (FT-IR, Thermofisher Scientific, Waltham, MA, USA).The surface morphology of the Co 3 (PO 4 ) 2 materials were examined by FE-SEM (SEM, JSM-6380, JEOL, Peabody, MA, USA) and HRTEM (TEM, JEOL) The elemental compositions and chemical bonding of the synthesized Co 3 (PO 4 ) 2 nanospheres JEOL-6330, USA were investigated by XPS (XPS, Thermo Scientific).The textural properties of the BET surface area, pore size, and pore volume were assessed on the basis of nitrogen adsorption-desorption isotherms using a NOVA 2200e analyser (Quantachrome Instruments, Boynton Beach, FL, USA).

Electrochemical Analysis
All of the electrochemical analyses were conducted in a conventional three-electrode system using a potentiostat (VSP-0478, Biologic, Seyssinet-Pariset, France, with ECLAB software, 10.1, Biologic, Seyssinet-Pariset, France, 2011) in a 1.0 M KOH (pH = 13) solution.The electrodes consisted of Pt foil, a saturated calomel electrode, and a Co 3 (PO 4 ) 2 electrocatalyst ink drop-casted Au electrode as the counter, reference, and working electrodes, respectively.The Co 3 (PO 4 ) 2 electrocatalyst ink was prepared by dispersion of a fixed amount of the Co 3 (PO 4 ) 2 electrocatalyst dispersed in 0.4 mL of 2-propanol, 0.1 mL of water, and 40 µL of 5 wt % Nafion under ultrasonication.The obtained electrocatalyst ink (0.07 to 0.28 mg/cm 2 ) was drop-casted on the Au electrode and left in air to dry for approximately 1 h.

Conclusions
In summary, we successfully demonstrated a facile synthesis of Co 3 (PO 4 ) 2 nanospheres by a microwave-assisted method and utilized as an electrocatalyst for the oxidation of methanol.The phase formation, morphological surface structure, elemental composition and textural properties of the sample was examined by various physicochemical characterization methods, such as powder XRD, FT-IR, FE-SEM, HRTEM, XPS, and BET surface analyses.The detailed electrochemical analyses show that the Co 3 (PO 4 ) 2 nanosphere-modified electrode exhibited an admirable electrocatalytic activity for the oxidation of methanol in alkaline media.An extended investigation was made for the methanol oxidation by varying the experimental reaction parameters, such as catalyst loading, methanol concentration, and long-term stability for electro-oxidation of methanol.The superior electrocatalytic activity of Co 3 (PO 4 ) 2 should be attributed to its good surface morphological structure and high number of active surface sites.These results demonstrate that the Co 3 (PO 4 ) 2 nanospheres could be one a low-cost, more abundant, and a competent electrocatalyst for direct methanol fuel cells.

Figure 9 .
Figure 9. CVs of a Co3(PO4)2 nanosphere-modified electrode at 50 mV/s, (a) under various concentration of methanol solution, and (b) the plot of anodic methanol oxidation current density vs. methanol concentration at 1.65 V vs. RHE.

Figure 9 .
Figure 9. CVs of a Co 3 (PO 4 ) 2 nanosphere-modified electrode at 50 mV/s, (a) under various concentration of methanol solution, and (b) the plot of anodic methanol oxidation current density vs. methanol concentration at 1.65 V vs. RHE.

Table 1 .
Specific and mass activities of nickel-and cobalt-based catalysts for methanol oxidation in alkaline media.

Table 1 .
Specific and mass activities of nickel-and cobalt-based catalysts for methanol oxidation in alkaline media.