- Nano Express
- Open Access
Enhanced performance of a novel anodic PdAu/VGCNF catalyst for electro-oxidation in a glycerol fuel cell
© The Author(s). 2017
- Received: 9 October 2017
- Accepted: 2 November 2017
- Published: 25 November 2017
This study presents a novel anodic PdAu/VGCNF catalyst for electro-oxidation in a glycerol fuel cell. The reaction conditions are critical issues affecting the glycerol electro-oxidation performance. This study presents the effects of catalyst loading, temperature, and electrolyte concentration. The glycerol oxidation performance of the PdAu/VGCNF catalyst on the anode side is tested via cyclic voltammetry with a 3 mm2 active area. The morphology and physical properties of the catalyst are examined using X-ray diffraction (XRD), field emission scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. Then, optimization is carried out using the response surface method with central composite experimental design. The current density is experimentally obtained as a response variable from a set of experimental laboratory tests. The catalyst loading, temperature, and NaOH concentration are taken as independent parameters, which were evaluated previously in the screening experiments. The highest current density of 158.34 mAcm−2 is obtained under the optimal conditions of 3.0 M NaOH concentration, 60 °C temperature and 12 wt.% catalyst loading. These results prove that PdAu-VGCNF is a potential anodic catalyst for glycerol fuel cells.
- Glycerol fuel cell
- Anodic catalyst
- Palladium-gold alloy
Conventional sources of energy, such as fossil fuel, are limited and will someday be depleted. Though the consumption of fossil fuels remains a necessity, the combustible materials we use as fuel cannot be replaced quickly enough to meet future energy demands [1, 2]. Fuel cell is a promising renewable energy technology that combines hydrogen and oxygen to produce electricity, heat and water. Previously, hydrogen has been used as the basic fuel for fuel cells. Unfortunately, the difficult handling and storage of hydrogen require further research to replace hydrogen with liquid fuel as an energy carrier and to deliver hydrogen to a fuel cell .
In early research, methanol was the most common fuel used in fuel cells because of its high energy density and simple molecular structure. However, the main focus has shifted towards environmentally friendly materials. Methanol is therefore not applicable as a fuel because of its high toxicity . Additionally, as the fuel supplied to the anode, methanol shows the limitations of inefficient oxidation, low open-circuit potential, and crossover from the anode to the cathode . Therefore, to avoid the problems of methanol, glycerol has become a promising candidate for use in fuel cells. The abundance of glycerol, which is a major product of biodiesel, and its high energy density and low toxicity make this alcohol a good alternative for fuel cell applications .
The complex molecular structure of glycerol and the numerous intermediate species in the oxidation process are the primary barriers preventing the use of glycerol in fuel cells. Therefore, the choice of catalyst and reaction conditions is important to ensure the desired outcome. An alkaline medium, rather than an acidic medium, has been used for glycerol oxidation to overcome kinetic constraints during the oxidation reaction . At the anode, the catalyst provides a foundation to convert the chemical energy of the fuel into electrical energy. Since palladium-based materials are efficient anodic materials in alkaline media, bimetallic PdAu nanoparticles supported on vapor-grown carbon nanofibres (VGCNF) are used as catalysts for glycerol oxidation in this study. The properties of PdAu nanoparticles themselves, which have a high tendency to agglomerate, make the use of a catalyst support very important for improving the performance, utilization, and lifetime of the catalyst . Furthermore, in addition to their mechanical strength and surface area in the range of 10–200 m2 g−1, VGCNF have a unique structure with large number of edges in the lattice and basal regions, which provides a surface for metal-support interactions [9, 10]. The presence of VGCNF as a support material may improve both the dispersion of the metal catalyst and the electrocatalytic performance .
The dependence of the electro-oxidation of alcohol on the electrolyte temperature and NaOH concentration has been investigated in several studies. Tripković, Štrbac, and Popović  noted that increasing the temperature from 295 to 333 K increased the MOR activity of Pt and PtRu catalysts. Habibi and Razmi  studied the effect of NaOH concentration in the range of 0.5 M to 6.0 M and temperature in the range of 25 °C to 80 °C for prepared Au, Pd and Pt nanoparticles supported on a modified carbon ceramic electrode (CCE). The authors reported that the NaOH concentration and temperature directly influenced the oxidation of glycerol. The catalyst loading also affects the alcohol oxidation performance. Basically, reducing the effect of catalyst loading on alcohol oxidation, especially for complex molecules such as glycerol, is a significant challenge. Many studies have  developed 10 wt.% to 20 wt.% Pd/C and PdAu/C metal catalyst for the oxidation of ethanol and glycerol. The complexity of polyalcohols, such as ethanol and glycerol that involve many intermediate reaction mechanisms during oxidation, makes it difficult to use lower catalyst loadings.
These observations inspired this optimization study on the reaction conditions of glycerol oxidation. The effects of electrolyte temperature, NaOH concentration and catalyst loading on the performance of glycerol oxidation using PdAu/VGCNF were analyzed by response surface methodology (RSM). As a result, a predictive model was generated from the experimental data by varying one parameter at a time. RSM is an applied statistics technique for experimental design that is used to strategically plan and execute experiments and thereby reduce the number of experiments required to optimize the operational conditions in glycerol oxidation. RSM is a collection of mathematical and statistical techniques based on the fit of a polynomial equation to the experimental data [14, 15]. The use of RSM is more practical because it may include interactive effects among variables and will eventually depict the overall effects that the parameters have on the process . Very limited research has been performed on the operational conditions of the alloyed electrocatalyst. In addition, the RSM optimization of the half-cell performance for glycerol oxidation in alkaline medium using the PdAu/VGCNF catalyst has never been studied. Most studies have focused on the performance of a single cell. However, optimization of the parameters in a half-cell test may provide a benchmark that could be applied to single-cell operation.
Materials and chemicals
All precursor metal salts and chemical reagents, such as gold(III) chloride trihydrate (HAuCl4·3H2O), palladium chloride (PdCl2), trisodium citrate (Na3Ct), sodium borohydride (NaBH4), carbon nanofibres, sodium hydroxide, glycerine, 2-propanol, and 5 wt.% Nafion solution, were purchased from Sigma-Aldrich/USA.
For the physical analysis of the electrocatalysts, techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) spectroscopy and transmission electron microscopy (TEM) were used to examine the electrocatalyst crystallization, structure, morphology, elemental composition, size, and atomic distribution. XRD is used to identify the phase of crystalline materials. The instrument used in this work is a Bruker D8 Advance diffractometer equipped with a CuKα radiation source at 40 kV and 40 mA. Scanning of the electrocatalyst is performed at a rate of 2° min−1 between 30° and 90°. The Scherrer equation is used to determine the size of the crystalline particles in the powder. Topographical and elemental information for the nanostructured catalyst was obtained using a Gemini SEM 500 field emission scanning electron microscope equipped with an energy-dispersive X-ray spectroscope that can provide three-dimensional images as well as provide information on the elemental composition of the sample under analysis. Transmission electron microscopy (TEM) was performed with a Philips CM12 microscope operated at 120 kV. The sample catalyst was placed in ethanol in an ultrasonic bath for 30 min before analysis.
The methodological approach to synthesize the electrocatalyst used in this study is a mixed technique based on reduction and impregnation . This is the simplest method that allows the formation of a PdAu bimetallic alloy supported on vapor grown carbon nanofibres (VGCNF). The electrocatalyst synthesis started with 2 ml PdCl2 (0.05 M) mixed with 7 ml gold(III) chloride trihydrate (HAuCl4·3H2O) (0.012 M). The mixed solution was added dropwise to a certain amount of trisodium citrate (0.5 M). Trisodium citrate acts as a stabilizing agent to control the aggregation of the nanoparticles by lowering the surface tension between the solid particles and the solvent. Subsequently, the mixed solution was added dropwise to a stirred VGCNF slurry (isopropanol + DI water) and stirred for 3 h. Reduction of the metal precursors was carried out using an excess amount of freshly prepared ice-cold (0.5 M) sodium borohydride (NaBH4), and the solution was stirred overnight. A longer reaction time allows sodium borohydride, with its strong reducing ability, to react with the products. The molar ratio of NaBH4 to metal ions is 5 to 15, which provides a better catalyst dispersion and surface composition of the PdAu bimetallic alloy nanoparticles. The solution was kept under magnetic stirring overnight, filtered, washed with DI water several times to remove all of the solvent and dried at 80 °C for 10 h. In the preparation of the electrocatalyst PdAu bimetallic alloy supported on VGCNF, the metal loading was varied between 10 wt.% and 30 wt.%.
Cyclic voltammetry tests
Cyclic voltammetry experiments were performed for the electrochemical analysis of the electrocatalyst. Cyclic voltammetry measurements were performed using an Autolab (PGSTAT101) electrochemical workstation at room temperature. The catalyst ink was prepared by dissolving 5 mg of electrocatalyst in a mixture of distilled water, isopropyl alcohol, and 5 wt.% Nafion®. A 2.5 μl aliquot of the electrocatalyst ink was deposited on a glassy carbon electrode using a micropipette and then left to dry at room temperature. The electrochemical characterization of the electrocatalysts was performed by a cyclic voltammetry (CV) test over the potential range of − 0.8 to 0.4 V in 1 M NaOH at a scan rate of 50 mVs− 1 in 0.5 M glycerol/0.5 M NaOH solution. The concentration and temperature of the NaOH electrolyte varied from 0.5 to 6.0 M and from 25 to 80 °C, respectively. Both solutions were de-oxygenated by bubbling with N2 at 200 ml min− 1 for 30 min before taking any measurement of the glycerol oxidation reaction.
Factors and level for response surface study
Low level (−1)
X1:NaOH concentration, M
X3: Metal catalyst loading (%)
Experimental matrix of central composite design runs using Design Expert 8.0 software
Run order no.
Current density (mA cm−2)
X1:NaOH concentration (M)
X2: Temperature (°C)
X3: Catalyst loading
Physical characterization of the catalyst
Fit summary model statistic result
Adjusted R 2
Predicted R 2
ANOVA for current density of oxidation reaction of glycerol
Sum of squares
Probability > F
X1: NaOH concentration
X3: Catalyst Loading
Lack of fit
Adj R 2
Pred R 2
Performance of glycerol oxidation under different conditions of interactive parameters
Figure 7a shows the contour plot of the current density of the oxidation peak of the glycerol oxidation reaction at an electrolyte temperature of 40 °C. The highest current density that can be attained at this temperature is 150 mA/cm2, in contrast to the 130 mA/cm2 attained at 30 °C. Compared with the 30 °C electrolyte temperature, the metal loading can be in the range of 16–29 wt.% with a NaOH concentration ranging from 1.50 to 6.0 M to obtain a 130 mA/cm2 current density with an electrolyte temperature of 40 °C. However, using a NaOH concentration ranging from 5.0 to 6.0 M with a metal catalyst loading reduced by 2 wt.% (20–24 wt.%) would achieve the highest current density of 150 mA/cm2 at a 40 °C electrolyte temperature; a temperature of 30 °C can achieve a current density of only 130 mA/cm2. The change of the electrolyte temperature from 30 to 40 °C increases the current density at the oxidation peak of the glycerol oxidation reaction.
The contour plot of the current density of the oxidation peak of the glycerol oxidation reaction when the electrolyte temperature is further increased to 50 °C is shown in Fig. 7b. The highest current density at this temperature is 162 mA/cm2, but the area is small, and a metal catalyst loading and NaOH concentration of 21–22 wt.% and 5.75–6.0 M, respectively, are required. At an electrolyte temperature of 50 °C, using the same range of metal catalyst loading (20–24 wt.%) shifts the NaOH concentration by 0.5 M (4.5–6.0 M) to obtain the current density of 160 mA/cm2. The effect of temperature increases the current density to a high value for the same range of metal catalyst loading and NaOH concentration.
Figures 8a, b and 9a show the contour plots of the current density at the oxidation peak of the glycerol oxidation reaction at 60, 70, and 80 °C, respectively. In Fig. 8a, the current density has the highest value of 165 mAcm− 2 at 60 °C compared to that at 70 and 80 °C, which show current densities of 161 mAcm− 2 and 150.4 mAcm− 3, respectively. The PdAu/VGCNF catalyst obtains the highest current density at the oxidation peak of the glycerol oxidation reaction at 60 °C. At temperatures greater than 60 °C, the current density decreases. In Fig. 8a, the NaOH concentration required to obtain the highest current density is in the range of 5.0–5.5 M. However, obtaining a current density of 160 mA/cm2 requires a NaOH concentration as low as 3 M, which is the lowest concentration found in this study. The highest current density at an electrolyte temperature of 70 °C decreases to 161 mA/cm2, and the NaOH concentration ranges from approximately 4.0–5.0 M. Increasing the temperature to as high as 80 °C reduces the highest current density to 150.4 mA/cm2 as well as the NaOH concentration to the 3.5–4.0 M range.
The metal catalyst loadings needed to obtain the highest current density at temperatures ranging from 60 to 80 °C seem to be same, approximately 20–24 wt.%. Increasing the metal loading further only reduces the current density. The same conditions are also applied to other temperatures. Increasing the metal catalyst loading to over 24 wt.% may block the active sites for the glycerol oxidation reaction. The catalyst is active and allows the adsorption of glycerol onto the surface of the catalyst. However, the amount of catalytic metal on the support must be considered. A high catalyst loading will affect the thickness of the fuel cell catalyst layer due to the large volume of the carbon support. Furthermore, increasing the metal loading can contribute to the saturation of the electrochemically active surface area (EASA) . This may be due to the high likelihood of Pd aggregation, even in the presence of the support. Therefore, a high metal loading will increase the degree of nanoparticle aggregation and reduce the porosity, which may result in mass transport limitations and reduce the catalytic activity . If the temperature and catalyst loading are increased simultaneously, the decrease in the current density may cause the PdAu alloy particles to cluster, leading to limited mass activity because of the very rapid reaction rate of the redox trans-metalation reaction for the PdAu catalyst . Figure 9b shows the current density at the oxidation peak of the glycerol oxidation reaction with a metal catalyst loading of 20 wt.% as a function of electrolyte temperature and NaOH concentration. By setting the metal catalyst loading constant at 20 wt.%, the electrolyte temperature and NaOH concentration can be varied to obtain the optimum current density.
The increase in the current density results from the temperature of the electrolyte due to the improvement of the diffusion coefficients, mass transfer of the reactants and reaction kinetics. The glycerol molecules move faster when heat is introduced, thus enabling faster glycerol transport to the anode catalyst. However, increasing the temperature to above 65 °C did not have a significant effect on the current density; to be more precise, the current density became stagnant due to the formation of intermediate species, which might block the active sites and decay the performance of the catalyst . This is also observed for increased NaOH concentrations with a constant electrolyte temperature. The current density increases to 123.33 mAcm− 2 at a NaOH concentration of 6.0 M and a temperature of 25 °C, as shown in Fig. 9b. The current density increases because the increased OH− concentration in an alkaline electrolyte environment may give rise to greater OH− coverage on the catalyst surface. The presence of OH− facilitates the adsorption of glycerol on the catalyst active sites, and increasing the OH− concentration to a certain value will prevent the adsorption of glycerol on the catalyst sites and decrease the reaction rate of the glycerol oxidation . Figure 9b shows the decrease in the current density when the temperature and NaOH concentration approach 80 °C and 6.0 M, respectively. In general, the performance of the catalyst increases with increasing temperature and electrolyte concentration. However, at a certain point, these two operating conditions will have an adverse effect on the current density. Temperatures and NaOH concentrations that are too high will lead to a higher coverage of the active layer on the anode catalyst and a decrease in the cell performance . The highest current density is 164 mAcm− 2, recorded at a NaOH concentration of 6.0 M and a temperature of 60 °C.
Experimental and predicted current density in two different condition a) Optimum parameters for maximum current density. b) Maximum current density in minimum operational cost. c) Current Density before optimization and d) current density after optimization
NaOH concentration, M
Metal catalyst loading, (wt%)
Predicted current density, mAcm−2
Experimental current density, mAcm−2
Performance comparison with different electrocatalysts base on Pd in GOR in alkaline media with the scan rate of 50 mVs−1
Metal catalyst loading (wt%)
Concentration NaOH (M)
Current density (mAcm−2)
Pt/CCE Pd/CCE Au/CCE
Glycerol (0.5 M)
46.8 mAcm−2 51.8 mAcm−2 58.0 mAcm-2
PdmAu/C, Au/C and Pd/C
Ethanol (0.5 M)
10.6 mA μg−1
Methanol (1.0 M)
1.30 mA cm−2
Pd/C, PdAu/C 50:50, PdSn/C 50:50, PdAuSn/C 50:40:10 and PdAuSn/C 50:10:40
Glycerol (1.0 M)
PdAg (1:1) /C PdAg (3:1) / C PdAu (1:1)/C PdAu (3:1)/C
Glycerol (0.5 M)
Ethanol (1.0 M)
Glycerol (0.5 M)
Glycerol (0.5 M)
Response surface methodology using central composite design is a powerful method for the examination and optimization of multivariable procedures. In this study, the Design Expert RSM tool generated 20 experiments to analyze the effects of temperature, NaOH concentration and catalyst loading on the current density of the glycerol oxidation reaction via cyclic voltammogram testing. According to the F values in the analysis of variance (ANOVA) evaluation, the NaOH concentration and temperature of the electrolyte had significant effects on the response. High temperatures improved the reaction kinetics of the glycerol reaction. Meanwhile, a high NaOH concentration provided OH− ions that facilitated the glycerol oxidation reaction. The best expression or optimal conditions subject to the highest current density of 158.34 mAcm− 2 were found to be at a NaOH concentration, temperature and catalyst loading of 5.24 M, 60 °C and 12 wt.%, respectively. In conclusion, using RSM to optimize an analytical method verified and successfully determined the optimum conditions for glycerol oxidation when using PdAu/VGCNF as the catalyst.
The authors gratefully acknowledge the financial support given for this work by the Ministry of Higher Education (MOHE) under GSP/1/2015/TK01/UKM/01/1 and Universiti Kebangsaan Malaysia under DIP-2017-021.
Funding for the study was received from the Ministry of Higher Education (MOHE): GSP/1/2015/TK01/UKM/01/1 and Universiti Kebangsaan Malaysia: DIP-2017-021.
All authors read and approved the final manuscript.
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