Preparation and Characterization of Pd Nanoparticles Supported on Graphene-Based Anode Catalysts for Direct Methanol Fuel Cells

Abstract

Palladium (Pd) nanoparticles supported by graphene nanomaterials were prepared and tested in this work using methanol as the fuel. The synthesized nanoparticles were used as electrocatalysts for direct methanol fuel cell. The support materials were synthesized by modified Hummer’s method and subsequently doped with nitrogen using melamine. The electrocatalysts were synthesized using modified polyol method. The synthesis method of the electrocatalyst was further modified by adjusting the pH of the electrocatalyst from 12 to 13. The structural characterization of the support materials was carried out using Fourier Transform Infrared (FT-IR) spectroscopy and Brunauer-Emmett-Teller (BET) technique while that of the electrocatalysts was also done using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). The elemental analysis was carried out using energy dispersive spectroscopy (EDS) to validated the presence of N-doped in Nitrogen-doped graphene oxide (NGO) and NrGO support materials and the Pd loading. The electroactivity, electron kinetics and stability of the electrocatalyst towards methanol oxidation were evaluated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) respectively. The results showed that the modification of electrocatalyst by increasing the pH to 13 did not improve the activity of the electrocatalyst generally since the supported Pd catalysts synthesized by modified polyol method exhibited better electroactivity towards methanol oxidation than their pH 13 counterparts.

1. Introduction

The need for continuous fight against climate change which is one of the major consequences of consistent use of fossil fuel has been established. This exponential increase in fossil fuel consumption leading to increase in environmental pollution caused by emission of carbon monoxide has become a major concern. It has been reported by International Energy Agency (IEA) that in 2018, the global emission of CO₂ energy related has risen beyond 33 Gt CO₂ [1] of which over 10 Gt came from coal used in power generating plant, majorly in Asia. This was as a result of increase in energy demand despite an increase in renewable sources of 7% in 2018 which resulted to 0.5% increase in CO₂ emission for every 1% gain in economic output globally [2]. The United Intergovernmental Panel on Climate Change has recommended reduction of greenhouse gas emission by 50–85% by 2050 in order to mitigate the climate change [1]. The reduction in this hazardous emissions can be achieved by using renewable energy source such as methanol [3, 4]. Hence, the need to translate from fossil fuel based energy to alternative source of energy has become imperative [5, 6]. These have drawn the attention of government and researchers on how to develop, improve and commercialize greener alternative, renewable and sustainable sources of energy [6].

The major sources of renewable energy, which are wind and solar, are characterized by irregular and instability of weather condition and time of each day [7]. Methanol has been reported to be environmentally friendly which does not suffer from these weather conditions and can be used as a good fuel when synthesized from hydrogen by electrolysis and CO₂ from atmosphere, biomass or from exhaust of industrial processes [8, 9]. Therefore, its usage can reduce the climate change. Methanol can be CO₂ free if obtained from renewable sources. Methanol is a good substitute for oil and can be easily handled [10]. Furthermore, being a liquid at ambient condition of temperature and pressure, it ties the habit with renewable energy systems where fuel cells and electrolysers play major role [2].

China has been reported as the largest methanol producer in the world with about 70 million tons [11]. In 2018, methanol consumption was about 17.4 million tons while only road transportation consumed about 126 million tons of petrol and 156 million tons of diesel fuel [12]. The percentage of methanol usage around the world is given as: China- 58%; the rest of Asia-Pacific- 16%; Europe- 13%; Latin America- 2%; North America- 10%. The rate of methanol production in China has increased exponentially despite its reduction globally. The migration to methanol economy in China strengthened their energy security, air pollution, reduced CO₂ emission and enhance the added value of their domestic economy [1].

The use of methanol as transportation fuel in China has geometrically increased its demand in all sectors reaching up to 40% of its total methanol production despite the abundant of coal in order to ameliorate energy security and air pollution. This has invariably enhanced their energy dependence and reduction in harmful emission. Methanol vehicle pilot programme was carried out in China between 2012 and 2018 and the results showed that methanol is a viable transportation fuel with no technical, economic and safety challenges. Several thousands of vehicles are now using methanol as fuel [12]. This increase in methanol vehicle has improved their economic sustainability. This has also been extended to marine vehicles as the International Maritime Organization set new limit such that all sizes of ships need to use fuel that meet the 0.5% m/m SOx emission with effect from 1st January, 2020 for ship operating outside designated emission control areas [2].

Methanol is majorly converted into neat methanol-M100, methanol and petrol blend (M5, M10, M15, M30, M50 and M85), methanol based petrol, methyl tertiary butyl ether (MTBE), dimethyl ether (DME) and biodiesel [1].

1.1 Benefits of methanol based fuels

The use of methanol either as neat or blend fuel has major advantages over the conventional uses. Tian et al. [13] reported that M20 enhance the thermal efficiency of engines and reduction of CO, CO₂ and NOx emission [13]. Wang et al. [14] noted that M15 and M25 are more accepted than petrol [14].

Electrofuel, in which methanol is an example, has been reported to play a significant role in decarbonizing aviation [2]. Goldman et al. [15] reported five electrofuel including methanol for use in aviation and discovered that they can be good substitute for the conventional kerosene base fuels but higher structural loads and lower efficiency still remain as challenge [15].

Recently, there has been paradigm shift in methanol utilization. About 85% of methanol production was reportedly use in chemical industry in 2012. This was due to the fact that methanol exhibits some unique characteristics which placed it ahead conventional fuels for internal combustion engine. This include high octane rating, high latent heat, fast burning velocity, high compression ratio and high knock resistance which increase engine efficiency [16, 17]. Methanol has been a good fuel of choice mostly in motorsport due to its clean burning and less expensive nature, high performance, safety and high energy density [2] compared to other fuels [18, 19, 20].

This chapter therefore focuses on direct methanol fuel cells using methanol as a renewable fuel and modifying graphene supported Palladium catalyst as facilitator of methanol oxidation for easy transition from the use of fossil fuel to a sustainable renewable methanol fuel as energy source for the future.

1.2 Methanol in fuel cells

Methanol is used in fuel cells either directly in direct methanol fuel cells (DMFCs) or indirectly via methanol steam reforming into hydrogen-rich gas mixture in high temperature proton exchange membrane fuel cells (PEMFCs). This discussion is limited to DMFCs in this chapter.

1.2.1 Direct methanol fuel cells

Low-temperature fuel cells using hydrogen, methanol, ethanol and other fuel is a technology which has drawn the attention of many researchers because they serve as means of generating power by direct conversion of the fuel and oxygen into water, electrochemically [21, 22]. Direct methanol fuel cell is one of these new technology.

A DMFCs is a low-temperature PEMFCs that make use of liquid methanol as fuel. This methanol, which serves as the fuel, can be produced from biomass which is non-hazardous to the environment [23]. The operating principle of a DMFCs comprises five major porous layers which include anode gas diffusion layer (AGDL), anode catalyst layer (ACL), polymer electrolyte membrane (PEM), cathode catalyst layer (CCL) and cathode gas diffusion layer (CGDL) as shown in Figure 1. The methanol fed into the anode diffuses through the AGDL to ACL where it is oxidized as shown in Eq. (1). During the cell operation, Eq. (1) proceeds forwards to form carbon dioxide, protons and electrons [24].

Figure 1.

Schematic of a DMFCs during normal operation. Source: [24].

$${\mathrm{CH}}_3\mathrm{OH}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{CO}}_2+6{\mathrm{H}}^{+}+6{\mathrm{e}}^{-}$$E1

The reaction in the ACL occurs in three-phase boundary which include catalyst particles, carbon support and electrolyte (membrane). The electron produced at ACL are transferred through the carbon support to the AGDL where they move through the external circuit and converted to electric current while the remaining unconverted electrons move to the cathode side of the fuel cell. However, the proton generated are transferred through the ACL ionomer phase to the membrane. The membrane is impermeable to the electron and gaseous species. At the cathode, oxygen gas is being forced in as it diffuses through the CGDL to CCL where it is reduced to heat and water in the presence of electrons and protons as shown in Eq. (2) [24]. These prominent features enable DMFCs to be considered as a promising device to supply power in portable devices [5, 25, 26].

$$3{\mathrm{O}}_2+12{\mathrm{H}}^{+}+12{\mathrm{e}}^{-}\to 6{\mathrm{H}}_2\mathrm{O}$$E2

The net equation for DMFCs reaction can be summarized as:

$$2{\mathrm{CH}}_3\mathrm{OH}+3{\mathrm{O}}_2\to 2{\mathrm{CO}}_2+4{\mathrm{H}}_2\mathrm{O}$$E3

The aim of DMFCs research is to develop low cost, high performance and durable cells with maximum oxidation of methanol fuel that can power portable devices [27, 28]. A lot of research has been carried out with the intension of reducing the cost and increasing the performance of fuel cells using different strategies. Some of these strategies include reducing the electrocatalyst loading in fuel cell electrodes, developing novel nanostructured thin-film Platinum such as 3 M’s nanostructured thin film (NSTF) electrode, decreasing the electrocatalyst nanoparticles size, reducing Platinum dependence by developing metallic alloy either as binary or as ternary and Platinum-free electrocatalysts, improving electrocatalyst dispersion using novel fabrication methods, developing membrane electrode assembly (MEA) fabrication methods to enable better catalyst dispersion and utilization, using new techniques to increase mass transport at the fuel cells electrode surface, improving the performance of carbonaceous electrocatalyst supports and exploring novel non-carbonaceous electrocatalyst support materials [27, 28]. In contrary, the present DMFCs system is very expensive (mainly due to catalyst used) with low performance and less durability. One of the major factors to be considered in designing high performance and more durable DMFCs is the catalyst support materials [29, 30] since they have been discovered to reduce the cost, improve the catalytic activity by increasing the catalyst nanoparticle distribution and durability for maximum oxidation of methanol fuel in DMFCs if properly developed [31].

1.3 Support materials

At this juncture, it is imperative to mention that high performance of DMFCs also depend majorly on the properties of the support materials used [32]. Electrocatalyst support materials play a crucial role in enhancing electrocatalyst activity during DMFCs operation. Through their electronic and atomic structure, they provide a good surface area for homogenous dispersion, better particle size and also promote the stability of the catalyst nanoparticles [6, 28, 33, 34, 35, 36, 37]. Since the instability of the catalyst support materials results to detachment of catalyst nanoparticles from the support materials causing the loss of activity of the electrocatalysts, the support materials can therefore significantly influence the activity of the catalysts and prolong their stability [38, 39]. Therefore, the activity and stability of electrocatalysts are function of the type of support materials used [40] as supported metal catalysts have been discovered to show higher stability and activity compared to the unsupported ones [39].

A lot of research has been carried out on large number of carbon support materials. Due to their high availability and low cost, carbon black materials have been widely explored as support materials for Pt and Pt alloyed electrocatalysts in low-temperature fuel cells such as DMFCs [6, 21, 26, 35, 41, 42, 43, 44, 45]. Among the carbon black support materials developed include Vulcan XC-72, Black Pearls 2000, Acetylene Black, Ketjen Black and Mascorb and they all exhibit high surface area (>100 m²g⁻¹) and good electrical conductivity (>1Scm⁻¹). Among these carbon black support materials, Vulcan XC-72 with BET surface area of 250 m²g⁻¹, mesoporous and macroporous percentage of 54% and electric conductivity of 2.77 Scm⁻¹ has been reported to show a significant performance in fuel cell environment [6, 26, 35, 46]. Furthermore, carbon materials with high nanoarchitectural graphitic structures such as multi-walled carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) have also been critically examined. This is as a result of their unique features as they offer better crystalline structure, high electrical conductivity, excellent corrosion resistance with high level of purity. MWCNTs in particular, is of great interest because of the specific structural, mechanical and electrical properties they exhibit [21, 38]. Mesoporous carbons (MCs) which include ordered mesoporous carbons (OMCs) have also been extensively studied as support materials for Pt and Pt alloyed electrocatalysts [6, 21]. Compared to carbon blacks, mesoporous carbon materials possess higher surface area with little or no micropores which facilitate the high dispersion of the catalyst nanoparticles on their surface and their pores. This results in large effective surface area of the electrocatalyst with high catalytic activity. Mesoporous structure with mesoporous size of 2–50 nm enhance easy mass transport, producing high limiting current value [6, 47, 48, 49, 50, 51, 52].

Recently, research interest has also been diverted, towards prominent 2D graphene and its N-doped derivatives [53]. This attraction is due to their unique graphitic forms, high charger-carrier mobility (up to 105 cm²V⁻¹S⁻¹), super conductivity, ambipolar electric field effect, quantum Hall effect at room temperature, high mechanical strength (130 GPa) and high surface area (2600 m²g⁻¹) [54]. Graphene surface area contains enough oxygen functional groups which give it a better advantage over other support materials. This enables graphene to disperse any metal nanoparticles easily and efficiently. It also possesses the ability to remove a lot of accumulated carbon monoxide which act as a poison during the adsorption of the catalyst nanoparticles thereby increasing the electrocatalytic activity of the catalyst [55, 56, 57, 58, 59]. Moreover, N-doped graphene has also been discovered to be a good catalyst support material due to its ability to introduce chemically active sites for reaction and anchoring sites for metal nanoparticles deposition, modify electronic properties and give carbon materials a metallic character [60]. Doping of graphene with nitrogen, which serves as a strong metal-support link, facilitate reduction in CO_ads accumulation on the surface of the electrocatalyst, thereby increasing the catalyst poison tolerance, high electrocatalytic activity and long durability [54]. Other carbon supports that have been investigated as support materials for electrocatalysts include carbon gels (CGs), carbon nanohorns (CNHs), carbon nanocoils (CNCs), activated carbon fibers (ACFs) and boron-doped diamonds (BDDs) [61, 62].

Furthermore, since activity and stability of direct methanol fuel cells (DMFCs) are anchored on the strong chemical synergistic interaction between the catalysts and the supporting materials which determines the proper dispersion of the catalyst nanoparticles at low metal loading [6], ideal catalyst support materials should therefore contain the following features among others: sufficient electrical conductivity, large surface area, high resistance to electrochemical corrosion, suitable porosity and porous structure, strong stability in acidic or alkaline medium, good proton conductivity and crystallinity, good compatibility with electrodes, good water handling to avoid flooding and easy recovery of catalysts which all result into strong chemical synergistic interaction between the support and the catalyst nanoparticles as shown in Figure 2 [6, 21, 42, 63]. Carbon support materials have been reported to be the best choice as catalyst support due to their large specific surface area, strong and better corrosion resistance and relatively low price [26].

Figure 2.

Properties of an ideal catalyst support. Source: [63].

Based on the review of related literature, it is highly evident that support materials are important as they contribute immensely to the activity, stability and durability of catalyst and consequently enhance the performance of the fuel cells. Despite the new carbon support materials that have been explored, the DMFCs electrocatalysts still suffer from dissolution, agglomeration, detachment from support materials and corrosion of support materials as shown in Figure 3. These challenges between the properties of these novel support materials and their real application under fuel cell operating system still create gaps which need to be rectified. Therefore, there is a need to modify these support materials by optimizing their properties in respect to fuel cell practical working condition by considering selection of appropriate support materials, their combination ratio (for the hybrid supports), synthesis procedure, MEA preparation and their integration into fuel cell system.

Figure 3.

Schematic illustration of electrocatalysts degradation. Source: [31].

1.4 Carbon supported catalysts used in direct methanol fuel cells

Electrocatalysts has been noted to play a significant role in DMFCs architecture and have been extensively explored to enhance the rate of electrochemical reactions in order to get desirable results [64]. These catalysts are either used as anode catalyst where oxidation reaction occurs or as cathode catalyst where reduction reaction takes place. They could be developed as electrode itself or coated on the surface of the electrode. Platinum and Palladium are mostly used in DMFCs as pure metal doped on carbon support materials or as alloyed with other metals [29, 65, 66].

Platinum has been extensively used in DMFCs being the known most active metal for methanol oxidation reaction and oxygen reduction reaction among other pure metals when supported on a conductive carbon material [64, 65]. However, the activity for the methanol oxidation reaction of Pt metal alone is very low (Ermete [66]) as it suffers kinetic limitation and also readily poisoned by CO specie, a product of methanol oxidation at low temperature [67, 68]. This poisoning effect usually result to instability as well as reduction in DMFCs performance. It is also known that the corrosion of carbon black increases in the presence of Pt nanoparticles. This results to detachment of the Pt from the support and the agglomeration of the Pt nanoparticles [64]. Hence, the use of additional metal with Pt such as Ru, Ni, Co, and Mo as alloy has been developed [69, 70]. The bifunctional mechanism explains that the second metal supplies oxygen to oxidized the Pt-adsorbed methanol oxidation intermediate specie, while the electronic effect states that the second metal modifies the Pt electronic configuration, thereby weakening the adsorption of the methanol oxidation intermediate specie on Pt (Ermete [66]).

Significant efforts have also been made to develop new catalyst for DMFCs anode with little or no Pt metal and are able to tolerate poisoning by CO specie with fast kinetics [29]. In view of this, Pd has aroused notable interest as a substitute to Pt in electrocatalysts since it is more abundant in nature than Pt and exhibits the capacity to enhance the oxidation of several alcohols in alkaline media with significant electrochemical stability [29]. The attraction of Pd-based electrocatalyst emanated from the fact that, unlike Pt-based electrocatalyst, they can be highly active for oxidation of large variety of substrate in alkaline medium. The alloying of Pd with non- noble metal in catalytic architecture capable of rapidly and stably oxidizing alcohols in anode electrodes is expected to decrease the cost of the membrane electrode assembly (MEA) so as to boost the commercialization of DMFCs [29] but their performance was still found to be lower than expected [65].

Therefore, performance of different modified graphene support materials using Pd catalyst with the aim of improving the activity and stability of the electrocatalyst for maximum oxidation of methanol fuel has been investigated in this research. Palladium (Pd) is used in this study as alternative to Pt due to its lower poisoning effect, similar electronic configuration and lattice constant. It is also more abundant in nature than Pt and exhibits the capacity to enhance the oxidation of several alcohols in alkaline media with significant electrochemical stability [29].

2. Experimental section

2.1 Chemicals

All the chemicals used in this research are analytical purity grade and were used as received without any further purification. The chemicals used for the synthesis include ethanol (99%), Ethylene Glycol (99.9%), Methanol (99.9%), Nitric Acid (60%), Sulfuric Acid (90%) Potassium Hydroxide (85%), Sodium Hydroxide (98.87%) and Potassium Permanganate which were purchased from Kimix Chemical and Laboratory Suppliers, Cape Town, South Africa. Sodium Nitrate (99%), Melamine (99%), Graphite powder, Carbon nanofibers, 2-Propanol (99.5%) were purchased from Sigma- Aldrich while Hydrochloric Acid (32%) and Hydrogen Peroxide (50%) were purchased from B&M Scientific. Palladium Chloride was purchased from SA Precious Metal PTY Ltd. while Nafion solution was purchased from Ion Power Inc. The MWCNTs were bought from Carbon Nano-materials Technology Co. Ltd., Gargdong, Gyongju, Gyeonggi, South Korea with a width of ∼20 nm and a length of ∼10 μm. All synthesis was done using deionized water from the Milli-Q water purification system (Millipore, Bedford, MA, USA).

2.2 Synthesis of different support materials for palladium catalyst

Graphene oxide and reduced graphene oxide were synthesized using the modified Hummer’s method [35, 71]. As shown in Figure 4 1 g of natural flake graphite powder, 0.5 g of sodium nitrate and 50 mL of sulfuric acid were mixed at 0°C ice-water bath. 3 g of potassium permanganate, being a strong oxidizing agent, was added slowly into the solution every half an hour to oxidize the graphite powder, three times in total. After that 46 mL of hot deionized water was added into the suspension drop-wise. In this step the temperature was kept at 90°C and maintained for 1 hour. Subsequently to that, 20 mL of H₂O₂ was added into the suspension drop-wise to neutralize any unreacted potassium permanganate that remains [72]. The solution was taken to an ultrasonicator for 30 minutes with the power of 200 W. The suspension was centrifuged for 30 minutes at a rotation speed 3000 rpm to remove exfoliated Graphene oxide (GO) particles [73] and a mud-like material was obtained. The material was washed with deionized water and ethanol five times, respectively. Lastly, the product was dried at 80°C in an oven for 2 days. Subsequently, the GO was reduced by dispersing 1 g of graphene oxide in 1 litre of water by means of 1 hour ultrasonic treatment as shown in Figure 5. As a result, a homogeneous brown graphene oxide aqueous suspension was obtained. The pH of the suspension was adjusted to 10 by addition of ammonium hydroxide while 700 μL of hydrazine solution in THF was added into the suspension in drops as a reducing agent [74]. The suspension was then refluxed at 80°C for a period of 24 hours. A black flocculent substance gradually precipitated out of the solution. The product was obtained by vacuum filtration process. Finally, the resulting black product was washed with methanol and ultrapure water, dried at 80°C for 24 hours in an oven and stored in vial. Thereafter, the GO, rGO and CNT were doped with nitrogen using melamine as precursor.

Figure 4.

Synthesis procedure of graphene oxide (GO) and nitrogen-doped graphene oxide (NGO).

Figure 5.

Synthesis procedure of reduced graphene oxide (rGO).

2.3 Synthesis of palladium catalyst using modified polyol method

About 0.4 g of the support materials were dispersed in a 15 mL of ethylene glycol under stirring conditions followed by a sonication in an ultrasonic bath for 15 minutes. To this dispersion, a solution of PdCl₂ in 15 mL of ethylene glycol was added and left under stirring for 15 minutes. The pH of the solution was adjusted with freshly prepared 2 M NaOH in ethylene glycol solution to pH ∼ 12 in the modified polyol method used for synthesizing monosupported, hybrid supported and binary catalysts. This was later modified by changing the pH from 12 to 13. The mixture was sonicated for 15 minutes to aid homogeneous adsorption of the metal precursor onto the surface of the support. For the reduction of Pd ions, the mixture was transferred into an oil bath and heated at 165°C for 6 hours consecutively under stirring and reflux conditions. After completing the reduction, the mixture was left under stirring overnight to cool down to room temperature and then filtered and washed with water. Finally, the catalyst was dried in an oven at 80°C for 24 hours in order to remove all water content and stored in vial [74, 75, 76].

3. Results and discussions

This section presents the results obtained from the various characterization carried out on all the prepared graphene support materials and their electrocatalysts using different appropriate techniques. In this study, the pH was adjusted from 12 to 13 and the results were compared in order to know if pH 13 electrocatalysts will perform better than pH 12 electrocatalysts. First, the energy dispersive X-ray spectroscopy (EDS) coupled with the scanning electron microscopy (JOEL JSM-7500F Scanning Electron Microscope, Mundelein, II, USA) was used to evaluate the Pd metal loading and was found to be 37.67% which was the same for all the synthesized graphene supported Pd catalysts.

3.1 Surface characterization

Fourier Transform Infrared (FT-IR) and Brunauer-Emmett-Teller (BET) were used for the surface characterization of the synthesized carbon support materials while X-ray diffraction microscopy (XRD) and high-resolution transmission electron spectroscopy (HR-TEM) were used for the electrocatalysts.

3.1.1 Fourier transform infrared spectroscopy of graphene support materials

In this section, the presence of carbonyl group, hydroxyl group, nitrogen for the N-doped support materials and other functional groups in all the synthesized graphene support materials were confirmed using Fourier Transform Infrared (FT-IR) Spectroscopy. The obtained spectra are shown in Figure 6. For FT-IR analysis of GO, the band around 1708 and 1049 cm⁻¹ were assigned to C=O and C▬O of carboxylic acid respectively while the band around 1228, 1582, 2988 and 1394 cm⁻¹ were assigned to C▬O alcohol, C=C aromatic, C▬H alkane and C▬H alkane (bend) respectively. The appearance of C=O and C▬O peaks of carboxylic acid in GO is an indication of the formation of GO from graphite powder by chemical oxidation [77]. In rGO spectra, the band around 1712, 3436 and 1196 cm⁻¹ were assigned to C=O carboxylic acid, strong peak of O▬H and C▬O of alcohol respectively while the band around 1564 cm⁻¹ was assigned to the C=C aromatic. The significant reduction in C=O peak of carboxylic acid, disappearance of prominent C▬O peak of carboxylic acid and appearance of O▬H peak of alcohol in addition to the C▬O peak of alcohol in rGO show the reduction of GO to rGO [78, 79]. Furthermore, the FT-IR spectra of NGO shows the C=O and C▬O bands of carboxylic acid with C▬N band of amine which were observed around 1716, 1050 and 1223 cm⁻¹ respectively while the band around 2988, 1580, 1394 and 780 cm⁻¹ were assigned to C▬H alkane, C=C aromatic, C▬H alkane (bend) and C▬H aromatic. The appearance of C▬N peak of amine which displaced the C▬O of alcohol in NGO spectra is an indication of the formation of NGO from GO. In addition, the spectra of NrGO shows a medium peak of O▬H band around 3414 cm⁻¹ which correspond to that of hydrogen bonded alcohol; 1728, 1564 and 1188 cm⁻¹ which correspond to C=O carboxylic acid, NO₂ nitro compound and C▬O alcohol respectively. The significant reduction in C=O peak of carboxylic acid, disappearance of prominent C▬O peak of carboxylic acid and appearance O▬H peak of alcohol in addition to the C▬O peak of alcohol in NrGO show the reduction of NGO to NrGO [80] while the appearance of NO₂ peak still indicate the doping with nitrogen. All these observed bands are summarized in Table 1. The presence of N-doped in NGO and NrGO was validated using EDS as shown in Figure 7.

Figure 6.

The FT-IR spectra of synthesized GO, rGO, NGO and NrGO support materials.

Support Materials	Functional Groups	Observed bands (cm^−1)
GO	C=O Carboxylic acid	1708
	C▬O Carboxylic acid	1049
	C▬O Alcohol	1228
	C=C Aromatic	1580
	C▬H Alkane	2988
	C▬H Alkane (bend)	1394
rGO	C=O Carboxylic acid	1712
	C▬O Alcohol	1196
	O▬H Alcohol	3436
	C=C Aromatic	1564
NGO	C=O Carboxylic acid	1716
	C▬O Carboxylic acid	1050
	C▬N Amine	1223
	C=C Aromatic	1580
	C▬H Alkane	2988
	C▬H Aromatic	780
NrGO	C=O Carboxylic acid	1728
	C▬O Alcohol	1188
	NO2 Nitro compound	1564
	O▬H Hydrogen bonded alcohol	3414

Table 1.

Observed FT-IR spectra for synthesized graphene support materials.

Figure 7.

The EDS spectra of synthesized (a) Pd/GO (b) Pd/rGO, (c) Pd/NGO, (d) Pd/NrGO.

3.1.2 Brunauer-Emmett-Teller of graphene support materials

The specific surface area, pore volume and pore size of the prepared graphene support materials were also investigated using Brunauer-Emmett-Teller (BET) as presented in Table 2. Surface area measurements were taken from the support materials to first evaluate the surface area of the carbon support materials used. Among the prepared graphene support materials, NGO showed the highest surface area, pore volume and pore size of 41.92 m² g⁻¹, 0.05 cm³/g and 308.50 Å respectively. Since the performance of catalysts increases with increase in the support surface area and pore volume, the catalyst must therefore be supported with a high surface area and pore volume support materials for proper dispersion of the catalyst nanoparticles which aids the catalyst activity and make low catalyst loading feasible for fuel cell operations [21]. Figures 8 and 9 show the adsorption–desorption and pore distribution graphs of synthesized graphene support materials respectively.

Support Materials	Surface Area (m^2/g)	Pore Volume (cm^3/g)	Pore size (Å)
GO	9.20	0.03	67.50
rGO	3.36	0.02	277.23
NGO	41.92	0.05	308.50
NrGO	6.46	0.03	173.76

Table 2.

The BET surface area, pore volume and pore size of synthesized graphene-based support materials.

Figure 8.

Adsorption–desorption graphs of synthesized GO, rGO, NGO and NrGO.

Figure 9.

Pore distribution graphs of synthesized GO, rGO, NGO and NrGO.

3.1.3 X-ray diffraction of graphene supported palladium catalysts

The crystallinity and crystallite size of graphene supported Pd catalysts were determined using XRD spectra and classical Debye-Scherrer equation respectively as stated in Eq. (4).

$$\mathit{d}=\raisebox{1ex}{$\mathit{k\lambda }$}\!\left/ \!\raisebox{-1ex}{$\mathit{\text{βCosϴ}}$}\right.$$E4

where d is the crystallite size, K is the Scherrer constant which also depends on the crystal shape and the diffraction line indexes, λ is the X-ray wavelength which is equal to 0.154 nm, β (2ϴ) in radian is the width of the peak (full width at half maximum, (FWHM) or integral breadth) after correcting for instrumental peak broadening and ϴ is the Bragg angle [81].

The sharpest and the most intense peak of all the prepared Pd catalysts appeared around 40° 2-theta scale which is indexed as (111). This peak was used to determine the crystallite size of all the electrocatalysts. The XRD graphitic pattern of prepared graphene supported Pd catalysts synthesized by modified polyol method and the modified counterparts at pH 13 show five diffraction peaks at 2-theta value around 40.0276°, 46.5107°, 68.0866°, 81.9789° and 86.8841° and are indexed to the (111), (200), (220), (311) and (222) crystal plane of Pd face-centred cubic (fcc) crystallographic structure as shown in Figures 10 and 11 respectively [60, 82, 83]. The broad peak located at approximately 25° 2-theta scale on the other hand corresponds to the plane (002) of carbon [60, 84, 85]. The XRD spectra confirmed that all the graphene supported Pd catalysts are crystalline as illustrated in Figure 10. This is also corroborated with selected area electron diffraction (SAED) as shown in Figure 12. The better the crystallinity, the lower the ohmic resistance and the better the electron flow [86]. The summary of particle size and crystallite size of the graphene support Pd catalysts synthesized by modified polyol method is illustrated in Table 3.

Figure 10.

XRD spectra of graphene supported Pd catalysts synthesized by modified polyol method.

Figure 11.

XRD spectra of modified counterparts of graphene supported Pd catalyst.

Figure 12.

Selected area electron diffraction (SAED) of graphene supported Pd catalysts synthesized by modified polyol method: (a) Pd/GO (b) Pd/rGO (c) Pd/NGO (d) Pd/NrGO.

Electrocatalyst	Particle size (nm) HR-TEM	Crystallite size (nm) XRD
Pd/GO	5 ± 1.6	5.5
Pd/rGO	19 ± 1.0	19.0
Pd/NGO	5 ± 1.2	5.8
Pd/NrGO	12 ± 1.0	12.8

Table 3.

The particle size and crystallite size of the graphene support Pd catalysts synthesized by modified polyol method.

In case of the modified counterparts of graphene supported Pd catalysts at pH 13, the XRD spectra also revealed their crystalline structures too with the sharpest and most intense peak indexes as (111) at 40° 2-theta scale as shown in Figure 11. The selected area electron diffraction (SAED) indicated in Figure 13 also corroborate the crystallinity of all the modified electrocatalysts. The particle size and crystallite size of the modified counterparts of graphene supported Pd catalysts is illustrated in Table 4.

Figure 13.

Selected area electron diffraction (SAED) of modified counterparts of graphene supported Pd catalysts: (a) Pd/GO, (b) Pd/rGO, (c) Pd/NGO, (d) Pd/NrGO.

Electrocatalyst	Particle size (nm) HR-TEM	Crystallite size (nm) XRD
Pd/GO	5 ± 1.0	6.0
Pd/rGO	6 ± 1.2	6.2
Pd/NGO	5 ± 0.7	5.9
Pd/NrGO	5 ± 0.6	5.8

Table 4.

The particle size and crystallite size of the modified counterparts of graphene supported Pd catalysts.

3.1.4 The high-resolution transmission electron spectroscopy of graphene supported palladium catalysts synthesized by modified polyol method

Figures 14 and 15 show the nanomorphological structures of all the synthesized graphene supported Pd catalysts examined using HR-TEM with their frequency distribution from 50 randomly selected nanoparticles. For graphene (GO, rGO, NGO and NrGO) supported Pd catalysts synthesized by modified polyol method, the images revealed a homogenous with relatively small particle size (since nanoparticles usually show a nanodimensional size of 1–100 nm [87]) distribution which ranges between 5 and 19 nm as shown in their respective histograms in Figure 14.

Figure 14.

HR-TEM images with their respective histograms for graphene supported Pd catalysts synthesized by modified polyol method: (a) Pd/GO (b) Pd/rGO (c) Pd/NGO and (d) Pd/NrGO.

Figure 15.

HR-TEM images with their respective histograms for modified counterparts of graphene supported Pd catalysts: (a) Pd/GO (b) Pd/rGO, (c) Pd/NGO and (d) Pd/NrGO.

In case of modified counterparts of graphene (GO, rGO, NGO and NrGO) supported Pd catalyst, the images also revealed a homogenous distribution with relatively small particle size distribution which ranges between 5 and 6 nm as shown in their respective histograms in Figure 15.

3.2 Electrochemical evaluation

3.2.1 Cyclic voltammetry

The electrochemical properties of graphene supported Palladium electrocatalysts synthesized by modified polyol method and their modified counterparts in alkaline (1 M KOH) solution were first examined by cyclic voltammetry (CV) with Pd loading of 0.02 mgcm⁻². The CV curves of each catalyst were obtained from the stabilized curve after scanning 20 cycles [88]. The cyclic voltammetry shows the adsorption/desorption peaks in the hydrogen region at negative potentials. As more negative potentials were applied, the reduction of H⁺ and the adsorption of H atoms become stronger:

$${\mathrm{H}}_{\left(\mathrm{aq}\right)}^{+}+{\mathrm{e}}^{-}+\text{site}\to {\mathrm{H}}_{\left(\mathrm{ad}\right)}$$E5

This process continued as electrode potentials became more negative until the formation of a H (ad) monolayer was achieved. Immediately the Pd surface was fully covered by hydrogen atoms, the adsorption of H₂ molecules occurred:

$$2{\mathrm{H}}_{\left(\mathrm{ad}\right)}\to {\mathrm{H}}_{2\left(\mathrm{ad}\right)}$$E6

These adsorbed hydrogen molecules came together to form hydrogen bubbles which left the Pd electrode surface when they have grown large enough:

$${\mathrm{nH}}_{2\left(\mathrm{ad}\right)}\to {\mathrm{nH}}_{2\left(\mathrm{g}\right)}+2\mathrm{n}-\text{sites}$$E7

At this period, a high cathodic potential was applied on the electrode and many free sites were exposed to the solution. Immediately the above reaction occurred at a high rate, the sharp cathodic current, known as the hydrogen evolution, increased. The formation of the H(ad) monolayer can be easily detected at the potential where the cathodic current increases rapidly. When the potential is reversed, the opposite process (anodic currents in the hydrogen region) occurs [89].

The oxidation peak of all the prepared electrocatalysts was not well pronounced [85] while a significant cathodic reduction peak which is attributed to the reduction of PdO produced on the forward potential scan was observed between −0.2 and −0.4 V [83] for all the prepared electrocatalysts. Among electrocatalysts synthesized by modified polyol method, Pd/NGO exhibited the most intense cathodic reduction peak with highest current density which implies that it provided better evidence for the widest electroactive surface area (ECSA) among the graphene supported Pd catalysts synthesized by modified polyol method as shown in Figure 16a [90, 91]. However, for the modified electrocatalysts, Pd/NGO exhibited the most intense cathodic reduction peak as shown in Figure 16b which implies that it provided better evidence for the widest electroactive surface area among the modified graphene supported Pd catalysts [90, 92].

Figure 16.

The cyclic voltammetry of (a) graphene supported Pd catalysts synthesized by modified polyol method (b) modified counterparts of graphene supported Pd catalysts in N₂ saturated 1 M KOH at scan rate of 0.02 vs⁻¹.

The ECSA values of all the graphene supported Pd catalysts synthesized by modified polyol method and their modified counterparts were determined by peak area of the cathodic reduction peak of PdO using the equation [83, 93]:

$${\text{ECSA}}_{\mathrm{Pd},\mathrm{cat}}\left({\mathrm{cm}}^2/\mathrm{mg}\right)=\raisebox{1ex}{$\mathrm{Q}\left(\mathrm{C}/{\mathrm{cm}}^2\right)$}\!\left/ \!\raisebox{-1ex}{$420\mathrm{\mu C}/{\mathrm{cm}}^2{\mathrm{L}}_{\mathrm{Pd}}\left(\mathrm{mg}/{\mathrm{cm}}^2\right)$}\right.$$E8

where Q (C/cm²) is the charge associated with the reduction peak of the catalysts in Coulomb, L_Pd (mg/cm²) is the working electrode Pd loading (0.02 mg/cm²) while 420 μC/cm² is the value for oxygen monolayer of Pd in Eq. (5) [83]. From the CV results of graphene (GO, rGO, NGO and NrGO) supported electrocatalysts synthesized by modified polyol method, it is clear that Pd/NGO have the highest ECSA value of 1.84 m²/g compared to other graphene supported Pd catalysts as indicated in Table 5. For the modified counterparts too, it is also evident from the CV results that Pd/NGO exhibited the highest ECSA value of 3.87 m²/g among graphene supported Pd catalysts as illustrated in Table 6.

Catalysts	Electroactive Surface Area (m^2/g)	Current Density (mA/cm^2) for MOR	Current Density (mA/cm^2) for Chronoamperometry
Pd/GO	1.60	3.45	0.07
Pd/rGO	1.24	1.02	0.03
Pd/NGO	1.84	7.38	0.14
Pd/NrGO	1.53	2.99	0.11

Table 5.

Comparison of ECSA with current densities (MOR and Chrono) of graphene supported Pd catalysts synthesized by modified polyol method as determined from the anodic sweep (−0.1 to 0.4 V) at scan rate of 0.02 vs⁻¹.

Catalysts	Electroactive Surface Area (m^2/g)	Current Density (mA/cm^2) for MOR	Current Density (mA/cm^2) for Chronoamperometry
Pd/GO	1.70	2.43	0.05
Pd/rGO	3.52	2.70	0.06
Pd/NGO	3.87	3.88	0.14
Pd/NrGO	3.78	4.88	0.14

Table 6.

Comparison of ECSA with current densities (MOR and Chrono) of modified graphene supported Pd catalyst as determined from the anodic sweep (−0.1 to 0.4 V) at scan rate of 0.02 vs⁻¹.

3.2.2 Methanol oxidation reaction

The electrocatalytic activity of the as-synthesized graphene supported Pd catalysts synthesized by modified polyol method and their modified counterparts towards methanol oxidation reaction (MOR) in alkaline (1 M KOH) solution in the presence of methanol was examined by cyclic voltammetry (CV) as illustrated in Figure 17a. In the forward scan, the oxidation peaks correspond to the oxidation of freshly chemosorbed species coming from methanol adsorption. The reverse scan peaks are basically associated with the removal of carbonaceous species which were not completely oxidized in the forward scan than the oxidation of freshly chemosorbed species [94]. The onset potential of graphene supported Pd catalysts synthesized by modified polyol method and their modified counterparts varies from one to another as summarized in Tables 7 and 8 respectively. After the anodic scan, the anodic current density declined sharply as a result of the formation of PdO on the electrocatalysts surface at high anodic potential. As the backward scan commenced, the PdO began to reduce and the catalysts surface is reactivated and methanol oxidation occurred again [95]. Among the graphene (GO, rGO, NGO and NrGO) supported Pd catalysts synthesized by modified polyol method, NGO supported Pd catalyst display the highest anodic peak current density while NrGO supported Pd catalyst display the highest anodic peak current density among the modified graphene supported Pd catalysts, which implies better electroactivity towards methanol electrooxidation on forward scan of negative sweep as illustrated in Figure 17a,b and shown in Tables 5 and 6 respectively. This enhanced performance of Pd/NGO as well as Pd/NrGO which also concur to their stability test, can be ascribed to their better electroactive surface area and incorporation of dopant nitrogen [22, 29, 91, 92, 96, 97, 98]. The nitrogen functional group on the surface of these support materials intensifies the electron withdrawing effect against the Pd and the decrease in electron density of Pd facilitate the oxidation of methanol fuel [90]. The N-dopant also serve as defect sites to enhance the nucleation of catalyst nanoparticles [96].

Figure 17.

The cyclic voltammetry curves of methanol oxidation on (a) graphene supported Pd catalysts synthesized by modified polyol method (b) modified graphene supported Pd catalysts in N₂ saturated 1 M MeOH +1 M KOH at scan rate of 0.02 vs⁻¹.

Electrocatalyst	Onset Potential (V vs. Ag/AgCl)	Anodic peak for forward scan If (mAcm^−2)	Anodic peak for reverse scan Ir (mAcm^−2)	If/Ir ratio
Pd/GO	−0.38	3.45	0.49	7.06
Pd/rGO	−0.42	1.02	0.05	21.79
Pd/NGO	−0.36	7.31	0.68	10.83
Pd/NrGO	−0.49	2.99	0.35	8.54

Table 7.

Results of the study of CVs of graphene supported Pd catalysts synthesized by modified polyol method in 1 M KOH + 1 M methanol (MeOH).

Electrocatalyst	Onset Potential (V vs. Ag/AgCl)	Anodic peak for forward scan If (mAcm^−2)	Anodic peak for reverse scan Ir (mAcm^−2)	If/Ir ratio
Pd/GO	−0.44	2.43	0.35	6.94
Pd/rGO	−0.45	2.70	0.25	10.76
Pd/NGO	−0.46	3.88	0.53	7.28
Pd/NrGO	−0.47	4.88	0.65	7.57

Table 8.

Results of the study of CVs of modified graphene supported Pd catalysts in 1 M KOH + 1 M MeOH.

The ratio of forward anodic peak current (I_f) to reverse anodic peak current (I_r) indicate the tolerance ability of electrocatalyst to accumulation of carbonaceous products and less poisoned. This ratio is the supplementary method used to determine the CO tolerance of the catalysts. All the prepared graphene supported Pd catalysts display higher ratio values in excess of 1 which are larger than those reported in literature [95]. Large value of I_f/I_r shows higher oxidation of methanol and better CO tolerance [93, 95, 99]. From the results shown in Tables 7 and 8, it is observed among graphene (GO, rGO, NGO and NrGO) supported Pd catalysts synthesized by modified polyol method and their modified counterparts that Pd/rGO exhibited the highest I_f/I_r ratio of 21.79 and 10.76 respectively. Therefore, Pd/rGO show the best activity towards complete methanol oxidation in both cases. This implies that those with lower activity towards complete methanol oxidation experienced CO poisoning which practically reduces their expected performance [85].

3.2.3 Electrochemical stability

The electrochemical stability of the synthesized graphene supported Pd catalysts was also tested by chronoamperometry at −0.3 V for 30 minutes. In all the current density-time curves of the graphene supported catalysts, the oxidation current density rapidly reduced in the first 64 seconds while in their modified counterparts, it rapidly reduced in the first 20 seconds which was followed by a slower decay until it attained a steady state. The high current displayed at the beginning of stability testing could be ascribed to the double layer charging between the interface of electrode/electrolyte [85]. The gradual decrease in current density with time which was significantly observed may be attributed to poisoning of the electrocatalysts and decrease in electroactive surface area as the stability test progresses [85]. After 30 minutes’ stability study in 1 M KOH + 1 M methanol solution, it was observed among graphene (GO, rGO, NGO and NrGO) supported Pd catalysts synthesized by modified polyol method that the chronoamperometric responses show a different electroactivity order to that experienced in methanol oxidation. The NrGO supported Pd catalyst show more stability than GO supported Pd catalyst while NGO and rGO supported Pd catalysts still exhibit highest and lowest stability respectively in the following order: Pd/NGO > Pd/NrGO > Pd/GO > Pd/rGO as shown in Figure 18a. This implies that Pd/NGO, among graphene supported Pd catalysts, still shown better stability than other synthesized electrocatalyst with current density 0.1398 mAcm⁻². This better stability in Pd/NGO, which also concur to the MOR result, can also be attributed to the better electroactive surface area and incorporation of nitrogen into the support materials. This influenced the good dispersion of Pd nanoparticles and the stability of the electrodes [90]. Also, Pd/NrGO display better stability among the modified graphene (GO, rGO, NGO and NrGO) supported Pd catalysts as shown in Figure 18b which also show similar MOR results [43, 63, 91, 92, 96, 98, 100, 101]. The current density for the stability test of graphene supported Pd catalysts synthesized by modified polyol method and their modified counterparts is illustrated in Tables 5 and 6 respectively.

Figure 18.

The chronoamperometry of (a) graphene supported Pd catalysts synthesized by modified polyol method (b) modified graphene supported Pd catalysts in N₂ saturated 1 M MeOH +1 M KOH at potential of −0.3 V.

3.2.4 Electrochemical impedance spectroscopy

The electrochemical impedance spectroscopy (EIS) revealed the thermodynamic properties of the as-synthesized electrocatalysts. It was used to explore the electrocatalytic kinetics regarding the methanol electrochemical oxidation. EIS is among the most effective techniques used to explore the electrochemical parameters of the electron/electrolyte interface [83, 102]. Figure 19 show the interfacial behavior of the prepared electrocatalysts in KOH electrolyte containing methanol at potential of −0.3 V vs. Ag/AgCl. An equivalent circuit was employed for fitting the Nyquist plots (inset) which include solution resistance (Rs), charge transfer resistance (Rct) and double layer capacitance (Qdl). Basically, each plot shows a semicircle in the high frequency related to charge transfer. Among the graphene (GO, rGO, NGO and NrGO) supported Pd catalysts synthesized by modified polyol method, NrGO supported Pd catalyst exhibited the least electrochemical impedance. This implies that Pd catalysts supported by this support material show better chemical kinetics than other synthesized Pd catalysts as indicated by Nyquist plot in Figure 19a as the charge transfer kinetic of Pd catalyst on this support material significantly improved which encourage mass transfer. This was also confirmed by its resistance charge transfer (Rct) value of 0.723 kΩcm² which was determined using Randels-Sevcik cell fitting under open circuit as illustrated in Table 9 which was also used for all other prepared electrocatalysts [91, 96, 98]. Furthermore, among modified graphene (GO, rGO, NGO and NrGO) supported Pd catalysts, NGO supported Pd catalysts showed the least electrochemical impedance. This also implies that Pd catalysts supported by this material show better chemical kinetics among their counterparts as indicated by Nyquist plot in Figure 19b and confirmed by its resistance charge transfer (Rct) value of 0.708 kΩcm² as reported in Table 10 [29, 92, 97].

Figure 19.

The electrochemical impedance spectroscopy of (a) graphene supported Pd catalysts synthesized by modified polyol method (b) modified graphene supported Pd catalysts in N₂ saturated 1 M MeOH +1 M KOH at potential of −0.3 V.

Electrocatalyst	Rct (kΩcm^2)	Rs (kΩcm^2)	CPE [Yo] (mF)	N (CPE Exponent)
Pd/GO	2.40	0.02	0.74	1.00
Pd/rGO	13.60	0.01	0.12	1.00
Pd/NGO	0.88	0.01	1.82	1.00
Pd/NrGO	0.72	0.06	0.22	1.00

Table 9.

Summary of electrochemical impedance spectroscopy of graphene supported Pd catalysts synthesized by modified polyol method.

Electrocatalyst	Rct (kΩcm^2)	Rs (kΩcm^2)	CPE [Yo] (mF)	N (CPE Exponent)
Pd/GO	3.37	0.03	0.47	1.00
Pd/rGO	3.43	0.03	0.46	1.00
Pd/NGO	0.71	0.03	0.61	1.00
Pd/NrGO	1.17	0.04	0.30	1.00

Table 10.

Summary of electrochemical impedance spectroscopy of modified graphene supported Pd catalysts.

4. Conclusion

Conclusively, graphene supported Palladium catalysts synthesized by modified polyol method (Pd/GO, Pd/rGO, Pd/NGO and Pd/NrGO) were compared with modified catalysts. (Pd/GO, Pd/rGO, Pd/NGO and Pd/NrGO), it was noted that the activity of the modified ones towards methanol oxidation did not improve. The graphene supported Palladium catalysts synthesized by modified polyol method at pH 12 showed better activity towards methanol oxidation and more stability than their modified ones synthesized at pH 13 in which Pd/NGO synthesized by modified polyol method at pH 12 was identified as the best. This better performance in graphene supported Palladium catalysts synthesized by modified polyol method than their modified ones may be attributed to better dispersion of catalyst nanoparticles on their support materials and lower pH.

This study reported on graphene supported Palladium catalysts synthesized by modified polyol method (Pd/GO, Pd/rGO, Pd/NGO and Pd/NrGO) at pH values of 12 and 13. The FT-IR results showed the presence of nitrogen for the N-doped, carbonyl and hydroxyl groups on all the graphene support materials while the BET results showed the surface area of 9.20, 3.36, 41.92 and 6.46 m²/g for GO, rGO, NGO and NrGO respectively. The XRD confirmed the crystallinity of the Palladium catalyst with average particles sizes of 5.5, 19.0, 5.8 and 12.8 nm for Pd/GO, Pd/rGO, Pd/NGO and Pd/NrGO for catalysts synthesized at pH 12. The HR-TEM results revealed that the Palladium nanoparticles were evenly distributed with little agglomerations. The EDS confirmed the presence of Palladium in the catalysts with the metal loading of 37.67%. The electrochemical characterization of the catalysts was done using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA). The CV results showed that the catalysts synthesized at pH 12 showed better activity towards methanol oxidation; the EIS and CA also revealed that catalysts synthesized at pH 12 showed better kinetics with low electrochemical impedance and better stability respectively than those synthesized at pH 13.

In conclusion, pH 12 is a suitable pH for the synthesis of graphene supported Palladium catalysts.

Acknowledgments

This research is funded by Tertiary Education Support Programme (TESP); Eskom Holdings SOC Limited, Reg. No 2002/015527/06; National Research Foundation (NRF), South Africa, grant number 120375 and HySA systems, UWC, South Africa. We sincerely appreciate the Physics Department (UWC) for HR-TEM analysis as well as iThemba Labs for XRD analysis.