Synthesis and Characterization of Gold-Palladium Nanoparticles Catalyst For Improved Hydrogen Fuel Cell Performance

Transcription

1 Synthesis and Characterization of Gold-Palladium Nanoparticles Catalyst For Improved Hydrogen Fuel Cell Performance Adam Bennett a, Helen Liu a, Allen Tran a, Likun Wang b, Miriam Rafailovich a,b* a,b Chemical and Molecular Engineering Department, Stony Brook, NY, 11794, USA Abstract Alternative energy sources are becoming increasingly more important in meeting current energy demands, due to the issues faced by conventional energy sources such as rising costs, political volatility, depletion of resources and carbon emissions. One promising renewable energy source is the proton exchange membrane (PEM) hydrogen fuel cell, which creates only water as a major byproduct. Currently, platinum (Pt) is the predominant catalyst being used for PEM hydrogen fuel cells. The total system cost is predominantly due to the high cost of Pt. Pt catalyst activity in the hydrogen fuel cell suffers from exposure to carbon monoxide (CO), which limits the viability of PEM fuel cells for vehicles and stationary applications. Gold (Au) has been investigated as a promising catalyst for the oxidation of CO to carbon dioxide (CO2), the addition of which to the PEM fuel cell can increase its longevity. However, due to the high cost of Au, an alternative catalyst could greatly reduce the total cost of a fuel cell system while maintaining high performance, making it a much more feasible option. This research looks at the use of Gold-Palladium (Au-Pd) nanoparticles as a catalyst for the improvement of hydrogen fuel cell performance. In this study the bimetallic Au-Pd catalyst were synthesized via the Brust Method and characterized using extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM) techniques. The subsequent testing on the catalyst was done at the fuel cell test station located at Stony Brook University in order determine its power density. Through the characterization and test station results it was determined that the synthesized Au-Pd catalyst had an alloy structure and produced roughly 15% more power than the control catalyst. Keywords: Energy, PEM fuel cell, Catalyst, Nanoparticles, Hydrogen fuel cell, Au-Pd 1. Introduction Fossil energy sources are currently facing not only political issues such as volatility and an unpredictable nature, but also environmental concerns of resource depletion and carbon emissions. Alternative energy will be vital for our increasing global energy demands and renewable energy sources such as fuel cells can play a key role in combating these problems. A fuel cell is a device that uses a fuel to convert chemical energy into useable electricity. Fuel cells are an attractive energy source due to the fact that they don t emit greenhouse gases and they have a high energy density and efficiency [1]. In a proton exchange membrane (PEM) fuel cell, a polymer membrane is held between the cathode and anode. This membrane allows protons to pass while preventing electrons from doing so. 92

2 Nafion, a copolymer, is commonly used as the membrane due to its high conductivity and mechanical durability [2]. An external wire is placed on the fuel cell connecting the electrodes and as the electrons build up they travel from the anode into the cathode producing an electric current. Water and heat is also produced during this reaction. As of now PEM fuel cells are one of the most promising alternative energy sources for transportation as well as having commercial and residential applications. the presence of an electrolyte (the membrane). The reaction occurring at the anode is referred to as the hydrogen oxidation reaction (HOR) and the reaction occurring at the cathode is called the oxidation-reduction reaction (ORR). Table 1. Hydrogen PEM Fuel Cell Reactions Anode (Oxidation) Cathode (Reduction) H2 2H + +2e - O2+2H + +2e - H2O Total Reaction H2+ O2 H2O Figure 1. Schematic of PEM Fuel Cell [3] Currently the most common type of fuel cell is the hydrogen fuel cell. Hydrogen is the most abundant element on earth and can also be generated by splitting a water molecule with a DC current through the process of electrolysis [1]. In the PEM hydrogen fuel cell (PEM HFC), hydrogen is flowed through the membrane on the anode side and reacts with oxygen on the cathode side. The reaction occurs in In a fuel cell, the catalyst properties are important in that they have a large effect on performance. The ratedetermining step in the electricity production of the fuel cell is usually the ORR at the cathode [4]. In order for the reaction to move forward, toward the formation of electricity and water the activation energy of the reaction must be surpassed. Catalysts are needed for fuel cells to lower the reactions activation energy. The catalyst is used at the cathode and anode of the fuel cell and works by promoting the HOR and the ORR. Different catalysts and supports are widely studied in order to find the most efficient and cost effective fuel cell for operation. Although current technology exists for PEM HFC applications, its wide spread implementation is not yet feasible due to the high overall cost and questions of durability. In order for the PEM HFC to become a viable option for vehicle and 93 2

3 stationary applications the costs needs to be greatly reduced [1]. Platinum (Pt) is the predominant catalyst used in the PEM HFC and is responsible for the bulk of the cost. The durability of Pt catalysts is also questionable due to carbon monoxide (CO) poisoning [5]. The effects of CO poisoning on the Pt catalyst are especially of interest, since even just 25 ppm of CO can reduce PEM HFC output by 50% [6]. Many sources of hydrogen gas come from natural gas reforming and thus contain a considerable amount of CO2, which can become CO while the PEM HFC is operating and thus poison the Pt catalyst [7]. Current research is geared towards the cost reduction and increasing efficiency and durability by addressing the effects of CO poisoning. A major challenge that remains is finding a suitable catalyst which can oxidize CO. The application of a suitable catalyst which can oxidize CO allows the utilization of cheaper hydrogen gas sources by reducing the purity requirement of the feed hydrogen gas. Nanoparticles are particles between 1 and 100 nm in size and are often seen to have increased catalytic activity due to their small size which prove a larger surface area (source). Gold (Au) and Palladium (Pd) nanoparticles have both been shown to have catalytic properties in CO oxidation. It has been shown that supported Au nanoparticles are extremely effective catalysts for oxidizing CO [6]. The catalytic activity of Au can even further be enhanced by incorporating a second metal as an alloy [5]. Pd has been a metal of interest to combine with Au, since it can add electrons to the system and thus increase catalytic activity. The most noticeable improvement in catalyst performance occurs with a goldpalladium core-shell nanoparticle conformation. The palladium atoms on the shell withdraw atoms from the gold core, shifting the d-band center of palladium such that the adsorption of O2 and O-O bond breaking is promoted. The negative charge on the palladium shell stabilizes oxygen atoms as they dissociate, lowering the energy barrier for O2 to dissociate [9,10]. It has been shown that synthesis methods greatly affect the size and morphology of bimetallic nanoparticles. Current methods to synthesize Pd bimetallic catalysts lead to polydispersity or clusters of nanoparticles. These methods work, however, make characterization difficult. The morphology of nanoparticles has a great effect on the catalyst activity [5]. Current research involving Au-Pd nanoparticle catalysts involve either core shell or alloy structure and are synthesized in various ways. A bimetallic core shell nanoparticle is composed of two phases, one metal in the core and the other surrounding it. A bimetallic alloy is a random mixture of the two metals [11]. Figure 2. Possible representative architectures of bimetallic nanoparticles (a) alloy, (b) coreshell, (c) cluster-on-cluster, (d) sub-shell, and (e) intermetallic [11] 94 3

4 In this research the bimetallic nanoparticle catalyst was synthesized via the Brust Method, which is known to produce small, high-surface area thiolstabilized nanoparticles through the reduction of the metal. The Langmuir- Blodgett (LB) trough was used to spread the synthesized particles over water in a trough that uses the surface tension to compress them onto the Nafion membrane. Bimetallic nanoparticles are extremely difficult to characterize so the purpose of this research is to determine whether the Au-Pd nanoparticles synthesized via the Brust Method have an alloy or core shell structure. Extended X- ray absorption fine structure (EXAFS) was conducted at the Stanford Synchrotron Radiation Lightsource (SSRL). EXAFS experiments are used in order to model the coordination environment around the absorbing metal atoms. Transition electron microscopy (TEM) was also done in order to determine the size and dispersity of the nanoparticles. The goal of this characterization is to determine what the exact structure of the synthesized Au-Pd catalysts are. After the catalysts were characterized they were tested on the hydrogen fuel cell test station at Stony Brook University to determine the power output of the fuel cell with the nanoparticle catalysts. This was done by creating polarization curves of the voltage against current and power against current, which allows us to compare the power output between a control and the Au-Pd catalysts. We hypothesize that PEM HFCs assembled with the membranes with the synthesized Au-Pd nanoparticles will have increased power output as compared to HFCS with just the platinum catalysts, thus improving the performance of PEM fuel cells. If successful this research can in turn find ways to improve the durability of the fuel cell system. 2. Experimental Section 2.1 Materials HAuCl4 was purchased from Sigma Aldrich (99%). K2PdCl4 was purchased from Sigma Aldrich (99%). 0.1 mg/cm 2 Pt Loading Electrode Catalyst Paper was purchase from Fuel Cell Store. 2.2 Experimental Methods Nanoparticle Synthesis In preparing the nanoparticle solution, mg (1 mmol) of tetrachloroaurate (HAuCl4) and mg of palladous potassium chloride (K2PdCl4) were dissolved in 36 ml of deionized water. Following, mg of tetraoctylammonium bromide (TOABr) was dissolved in 96 ml of toluene. This solution was added to the tetrachloroaurate and palladous potassium chloride mixture and then magnetically stirred for 20 minutes until the mixture separated into two distinct layers. 200 µl of dodecanethiol and mg of sodium borohydride (NaBH4) dissolved in 30 ml of deionized water were added to the twolayer solution, and magnetically stirred at room temperature for 3 hours. The aqueous layer was removed from the solution via separatory funnel and the remaining top layer was rotary evaporated until 5 ml remained. 200 ml of ethanol was added to the top layer solution and refrigerated overnight at 4ºC. The top solution was removed from top layer solution and the remaining sample was centrifuged at 5000 rpm for 10 minutes then washed with ethanol multiple times. The sample was dried in the vacuum desiccator for 2 days. 95 4

5 2.2.2 LB Trough Coating The Langmuir-Blodgett (LB) Trough was calibrated to less than 0.25 mn/m surface pressure prior to nanoparticle coating. The 212 Nafion membrane is placed on the platinum plate, attached to a hook in the center of the LB Trough. 100 µl of nanoparticle solution was added via micropipette to each side of the LB trough. The solvent from the nanoparticle solution was allowed 10 minutes to evaporate before starting the coating process. The target pressure was set to 5 mn/m with pushing rate of 6 mm/min Fuel Cell Test Station The fuel cell test station was operated at 60C. The 212 Nafion membrane, was wetted with deionized water and placed in a pre-fabricated membrane electrode assembly (MEA). On the fuel cell test apparatus (Fuel Cell Technologies, Inc., SFC-TS), 78 ccm of hydrogen gas was constantly flowed into the MEA. The cell was held at 6V for 1 hour to stabilize it, and then cycled between 0.5 A/cm 2 and 1 A/cm 2 9 times. The cell was then operated at 0.2 A/cm 2 for 6 hours to fully humidify the cell. Following the break-in procedure, the VIR software in LabVIEW was initiated to conduct performance tests on the cell. 2.3 Analytical Methods Transmission Electron Microscopy (TEM): The nanoparticle sample was subjected to TEM observations using the JEOL JEM 1400 Transmission Electron Microscope. A few drops of nanoparticle solution using a micropipette was diluted in a petri dish of toluene and placed on a TEM grid. Images were observed at 100kx and 200kx magnifications. High Resolution TEM was also performed on the nanoparticle sample Extended X-Ray Absorption Fine Structure (EXAFS): Extended X-Ray Absorption Fine Structure measurements were performed on the nanoparticle solution at Stanford Synchrotron Radiation Lightsource (SSRL). X-Rays of narrow energy resolutions were shone at the sample and the transmitted x-ray intensity was recorded. Dependent on sample thickness, absorption coefficient, and atom type a number of photons are absorbed by the sample. When the incident x-ray energy matches the binding energy of the electrons of an atom in the sample, the number of x-rays is increased and the transmitted x-ray intensity drops, resulting in the absorption curve. 3. Results and Discussion The TEM images were observed to be spherical nanoparticles in Figure 3. The nanoparticle size distribution is relatively similar, with TEM gridded AuPd nanoparticle size averaged at 2.02 nm and the LB trough AuPd nanoparticle size averaged at 1.86 nm. This suggests a high surface area for the nanoparticles. (a) 965

6 (b) (a) (c) (b) Figure 3. TEM Images of (a) Au, (b) Pd, and (c) AuPd Nanoparticles Figure 4. AuPd NP TEM Gridded TEM Size Distribution Figure 5. AuPd NP LB Trough TEM Size Distribution Figure 6. (a) HR TEM of AuPd NPs (b) magnified view of crystalline platelet structure From the HR TEM, it can be seen that the AuPd NPs have a crystalline platelet structure, which is suggested in literature to by the structure needed by nanoparticle catalysts in order to effectively oxidize CO. The platelet structure has good contact with the support, and has enough surfaces for the reaction to occur, since the oxidation reaction occurs on the edges and steps of the platelet, rather than the smooth surfaces [8]. 97 6

7 The XANES (Fig. 7) analysis shows that the Au nanoparticles samples prepared through conventional methods and LB trough were consistent with metallic Au foil, with lower-intensity features due to the presence of thiolstabilized NPs. The EXAFS spectra also shows that the gold NP samples were close to that of metallic gold, but showed lower amplitudes again due to the presence of Au-S bonds. Figure 7. XANES for AuNP Figure 8. EXAFS for AuNP Figure 9. Pd K-Edge XANES for thiolstabilized PdNPs Figure 10. Pd K-Edge EXAFS for thiolstabilized PdNPs The Pd K edge XAS spectra for the palladium nanoparticles are shown in figures 8 and 9. The XANES spectra for both samples are different from the XANES for metallic Pd. They do however resemble the XANES spectra for PdS very closely. Similarly the EXAFS spectra and the Fourier transformed EXAFS for the nanoparticles show a very good indication of a strong Pd S contribution by the low frequency oscillations and a maximum in the low wavenumber range. The metallic Pd is also present in the sample as shown by the 2 and 3 Å peak indicating a Pd Pd bond. The XAS comparison of the nanoparticle samples prepared with conventional methods versus the LB method show us there is no significant difference between the two. No difference between the samples was seen in agreement with the XANES that both samples had a large contribution of Pd S bonds. From EXAFS analysis, it can be seen that there are no Pd-Pd bonds in the AuPd NPs sample. This suggests that the AuPd NP is in an alloy configuration rather than a core-shell configuration, since a core-shell configuration would show both Au-Au bonds and Pd-Pd bonds. The final part of this research involved determining the power output of 98 7

8 Table 2: Structure parameters (coordination numbers N, interatomic distances R and disorder factors σ 2 ), obtained in fitting of experimental EXAFS data the synthesized Au-Pd nanoparticle catalyst. This was done at the fuel cell test station at Stony Brook University. 0.1 mg/cm 2 Pt loading was used on the electrode for the control and the Au-Pd catalyst to compare the power output. The membrane used was Nafion 212, which has a thickness of 50.8 micrometers. Hydrogen was flowed through the fuel cell at a rate of 78 CCM (cubic centimeter per minute). The operating temperature was set at 60 ⁰C. Au-Pd nanoparticle catalyst as opposed to just the control. The maximum power output of the Au-Pd catalyst was watts whereas it was only for the control. Approximately 14.18% more power was generated with the Au-Pd nanoparticle catalyst. Figure 12. Power against current for Au-Pd Catalyst with 0.1 mg/cm 2 Pt catalyst loading and control of only 0.1 mg/cm 2 Pt catalyst loading on electrode. Figure 11. Voltage against current for Au-Pd Catalyst with 0.1 mg/cm 2 Pt catalyst loading and control of only 0.1 mg/cm 2 Pt catalyst loading. From the polarization curves we determined that the hydrogen fuel cell had a higher maximum power output with the 9Table 3. Max power and current for Au-Pd Catalyst with 0.1 mg/cm 2 Pt catalyst loading and control of only 0.1 mg/cm 2 Pt catalyst loading on electrode. Catalyst Max Power (watts) Max Current (amps) Au-Pd Control

9 In the fuel cell hydrogen is oxidized at the anode and oxygen is reduced at the cathode producing water. Platinum catalyzes both of these reactions. These results found that the use of an Au- Pd nanoparticle catalyst catalyzes the fuel cell reactions at a better rate yielding a better power output. 4. Conclusions The majority of the current work on fuel cells is aimed towards their potential use in vehicles, PEM fuel cells being the most promising. In order for PEM fuel cells to become a viable option for vehicles and stationary applications the cost needs to be reduced and the durability needs to be increased. Fundamental research is needed for the fuel cell membrane and catalyst layer. The characterization techniques of EXAFS in conjunction with TEM showed that it is highly likely the catalyst we synthesized was an alloy. The XAS spectra showed us that the obtained coordination numbers and Pd Pd and Pd S distances are consistent. Therefore it can be concluded that the investigated samples contain both metallic nanoparticles, as well as the low molecular weight Pd-thiol complexes. The absence of Pd-Pd bonds in the EXAFS analysis of the AuPd NPs strongly indicates that the synthesized catalyst is in the alloy configuration. The synthesized catalyst had an average size of 1.98 nm indicating that the Brust method was effective was effective in synthesizing Au-Pd nanoparticles with an effective surface area. From the HR- TEM, it could be seen that the synthesized AuPd NP had the desired crystalline platelet structure, further indicating the efficacy of the Brust method. The research performed shows a promising result of the Au-Pd alloy catalyst in the PEM HFC. The Au-Pd alloy nanoparticles increased the output of the fuel cell showing that it is an effective catalyst for fuel cell reactions. The power output of the Au-Pd catalyst was approximately 14.18% higher than the control. Acknowledgements We gratefully acknowledge the financial support from the Department of Materials Science & Engineering and the Program in Chemical and Molecular Engineering at Stony Brook University through research funding. We also thanked the Advanced Energy Center (AERTC) to provide laboratory and equipment and SSRL to give us analysis of our nanoparticles. References [1] Department of Energy Fuel Cell Technologies Office Multi-year Research, Development and Demonstration Plan [2] Wang, Y., Chen, K. S., Mishler, J., Cho, S. C., & Adroher, X. C. (2011). A review of polymer electrolyte membrane fuel cells: Technology, applications, and needs on fundamental research. Applied Energy, [3] Jones, R. L. (2017, March 27). Energy Storage & Delivery. Retrieved from [4] Reda, M. R. (1970, January 01). The Rate Limiting Step (RLS) for the Oxygen Reduction Reaction at the Cathode of Polymer Electrolyte Membrane Fuel Cell. [5] Hutchings, G., & Kiely, C. (n.d). Strategies for the Synthesis of Supported Gold Palladium Nanoparticles with Controlled Morphology and Composition

10 Accounts Of Chemical Research, 46(8), [6] Baschuk, J.J., and Xianguo Li. International Journal of Energy Research 25, no. 8 (200): [7] Janssen, G.J.M., and N.P. Lebedeva. In Presented at the Conference: Fuel Cells Science and Technology vol. 2004, pp [8] Haruta, M. Gold Bull (2004) 37: 27. doi: /bf [9] Chen, D. et al. Core-shell Au@Pd nanoparticles with enhanced catalytic activity for oxygen reduction reaction via core-shell Au@Ag/Pd constructions. Sci. Rep. 5, [10] Staykov, A., Derekar, D., & Yamamura, K. (2016). Oxygen dissociation on palladium and gold core/shell nanoparticles. International Journal Of Quantum Chemistry, (20), [11] Scott, R. J. (2015). Rational design and characterization of bimetallic goldpalladium nanoparticle catalysts. Canadian Journal Of Chemical Engineering, 93(4),