Synthesis and characterization of iron nanoparticles on partially reduced graphene oxide as a cost-effective catalyst for polymer electrolyte membrane fuel cells

Partially reduced graphene oxide functionalized with Fe nanoparticles alone or combined with Au and Pt nanoparticles is synthesized and characterized, and their effects on Polymer Electrolyte Membrane Fuel Cell (PEMFC) power output and carbon monoxide resistance are tested. Samples were prepared with various combinations of metal nanoparticles to create a cost-effective catalyst. Transmission and scanning electron microscopy revealed metal nanoparticles embedded on graphene sheets, some with magnetic susceptibility. PEMFC tests exhibited power output that was >180% of the control in a pure H2 gas feed and 250% of the control in a H2 gas feed with 1000 ppm of CO. Introduction The polymer electrolyte membrane fuel cell (PEMFC) converts chemical energy from the redox reaction of H2 and O2 to electrical energy using a platinum catalyst supported on carbon black paper as an anode and cathode to drive the redox reaction; a polymer electrolyte membrane (PEM) (in this case, a Nafion membrane), which serves as a medium for the proton exchange between the anode and cathode; and a wire through which the electrons can travel. The platinum catalyst on the anode oxidizes hydrogen gas (H2) into electrons and protons. The protons can pass through the PEM but the electrons cannot and so they flow into the wire, producing electrical energy. At the cathode, oxygen gas (O2) joins with the protons and electrons from the anode to create H2O as the only product. PEMFCs have several limitations: (1) The high cost of the platinum catalyst to oxidize H2. (2) The low output power of only 1 V produced by a single PEMFC. To solve this problem, fuel cells are stacked on top of each other, resulting in an expensive and bulky power source. (3) The PEMFC’s susceptibility to carbon monoxide (CO) poisoning of the platinum catalyst through the reverse water shift gas reaction: CO2 + H2⇌H2O + CO CO2, readily available from the environment, can constantly react with H2 to form CO gas. [2] CO gets absorbed by the anode, poisoning and blocking the platinum catalysts from oxidizing H2, thus reducing power output. Current methods of preventing CO poisoning have flaws in their practicality. Mixing trace amounts of O2 gas with H2 gas to stimulate CO oxidation on the catalyst has been investigated, but this proved ineffective and tedious due to the need of constantly monitoring the air content. The same results were true when hydrogen peroxide was added to the H2 gas. [4] Operating the fuel cell at >100 °C to oxidize the CO causes the Nafion membrane to degrade, making this method unfeasible as well. Metal catalyst nanoparticles such as gold (Au), platinum (Pt), and palladium (Pd) have produced promising results but their noble metal precursor salts are costly. Aggregation of metal nanoparticles is also a major drawback, decreasing the surface area to volume ratio and causing metal particles to lose their unique properties that are only present on the nanoscale. Thus, the goal was to develop an inexpensive, durable catalyst that would resist aggregation, thereby maintaining its catalytic ability. In addition, the ideal catalyst would not only resist CO poisoning but even increase power output of the PEMFC, reducing the amount of fuel cell stacks needed to produce sufficient power. Iron (Fe) could be a possible alternative catalyst. Fe, like other transition metals, can serve as a catalyst because it has multiple oxidation states and can lend or withdraw electrons from a reactant, forming complexes with it. Fe is quite economical; costing about $0.50/lb versus the cost of Pt, $22,000/lb. Fe is already used as a catalyst, for example, in reactions such as the splitting of N2 in the Haber process to form ammonia. [12] However, its challenges still exist, which are to create uniform nanoparticles and prevent aggregation. Graphene shows potential for PEMFC use due to its extraordinary properties, especially its remarkable electron mobility. MRS Communications (2017), 7, 166–172 © Materials Research Society, 2017 doi:10.1557/mrc.2017.14 166▪ MRS COMMUNICATIONS • VOLUME 7 • ISSUE 2 • www.mrs.org/mrc https://doi.org/10.1557/mrc.2017.14 Downloaded from https://www.cambridge.org/core. IP address: 54.191.40.80, on 21 Aug 2017 at 14:03:51, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. The absence of an effective mass of the electrons, caused by the linear relation between energy and movement at the Dirac points in the Brillouin zone of the graphene, causes the photon-like behavior of the electrons and ballistic transport. These two characteristics of the electrons in graphene allow the electrons to theoretically reach an electron mobility of 200,000 cm/V•s. Thus, graphene may support electron mobility from the oxidation of hydrogen in the PEMFC. In addition, graphene’s theoretically high surface-to-mass ratio of ∼2600 m/g, made possible by its two-dimensional character, is also a desired property as it allows for more nanoparticles to attach to the graphene surface, preventing aggregation and providing more catalytic surface area for H2/O2 reactions. [14] This research incorporated Fe into partially reduced graphene oxide (prGO) by first mixing the Fe salt with graphene oxide (GO) and then reducing the mixture with sodium borohydride (NaBH4). The prGO sheets would prevent nanoparticle aggregation, yet their high surface area would still expose the particles to the reaction system. Graphene’s high conductivity and low resistivity may aid H2 oxidation. To test which of iron’s oxidation states would attach best to the sheets and work optimally as a catalyst, Fe-prGO was synthesized from three different precursors: Fe from FeSO4; Fe +3 from Fe2(SO4)3; and Fe 0 from Fe(CO)5. Fe catalysts, even in combination with trace amounts of Au or Pt, could still greatly reduce the amount of noble metals needed to increase the power output of the PEMFC and may help resist CO poisoning, replacing some of the susceptible Pt. Furthermore, only partially reducing GO will maintain its water solubility, serving as a substrate for Fe nanoparticles, decreasing their aggregation yet still exposing their catalytic surfaces on the prGO sheets. Experimental details GO paste was chemically synthesized using a modified Hummer’s method. A GO solution was created by dissolving 200 mg of GO in 150 ml of distilled water, sonicating for 15 min, and then centrifuging at 3000 rpm for 20 min. The supernatant was then mixed with 50 ml of ethanol (to enhance its compatibility with the hydrophobic 30% PTFE-treated carbon electrodes) and used for experimentation. To functionalize GO with metal nanoparticles, the metal salt was added to 15 ml GO aliquots for a salt concentration of 0.05 mM, stirred overnight, then reduced with NaBH4 the following day. Three different Fe precursors were tested: (1) ferrous sulfate (FeSO4, Fisher Scientific), (2) ferric sulfate (Fe2(SO4)3, Flinn), and (3) iron pentacarbonyl (Fe(CO)5, Sigma-Aldrich). The solutions created using Fe were named using their ideal oxidation states. Potassium tetrachloroplatinate (K2PtCl4, Sigma-Aldrich) and potassium tetrachloroaurate (KAuCl4, Sigma-Aldrich) were used, respectively, to functionalize GO with Pt and Au nanoparticles. One solution of GO was functionalized with a combined mixture of Fe(III), Au, and Pt salts. In solutions containing two metals, each metal was added at a concentration of 0.025 mM for a total final concentration of 0.05 mM. For solutions containing three salts, each metal was at a concentration of 0.015 mM, achieving a total concentration of 0.045 mM metals. 20 drops of Fe(CO)5 were added from a micro syringe to one 15 ml GO solution, which were then reduced together; and another test solution was made by adding 20 drops of Fe(CO)5 to 15 ml of an already partially reduced GO solution. Partial reduction of each GO solution was made by bringing the final concentration to 12 mM NaBH4, creating prGO that was still water-soluble but had less functional groups. Scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX) (LEO 1550, Zeiss, Germany) samples were prepared by drying solutions onto 1 cm × 1 cm silicon wafers with a miller index of 100. Transmission electron microscopy (TEM) (JEOL JAM 1400) samples were prepared by dropping solutions onto lacey copper grids (400 mesh carbon support film, Ted Pella Inc.). Raman spectra were collected for 10 s using 100% laser power and an 1800 lines/mm grating, which provided a spectral resolution of ∼1 cm−1 using a Renishaw inVia microRaman spectrometer with a 514 nm inline argon laser. Spectra were analyzed using the Renishaw Wire 4.0 software and the quadratic baselines were subtracted for all data; the integrated signal under the curves was normalized to 1. Vibrating sample magnetometry (VSM) (Microsense model 880) samples were prepared by drying solutions and scraping them into flakes. The PEMFC electrodes (FuelCellsEtc, CTM-GDE, PtC 20%) had an initial Pt loading of 0.1 mg/cm and a 30% PTFE treatment in the microporous layer, which gave the electrodes a highly hydrophobic character. To counteract this, the electrodes were exposed to UV-O3 for 10 min using a Bioforce Sciences Ultraviolet Radiation System to cleanse the electrode surface of organic contaminants and create a high-energy surface. The electrodes were then soaked in the metallized prGO solutions for 5 min before being dried overnight in a 50 °C oven. Each Nafion membrane (FuelCellsEtc., Nafion 117) was coated with its test solution using a KSV 5000 LangmuirBlodgett (LB) trough dipper immediately before PEMFC testing to maximize hydration. The membrane was immersed in a beaker of distilled water and 2 ml of test solution were carefully pipetted onto the water’s surface to evenly layer a surface film. The 25% ethanol solution’s lower density allowed the solution to float on top of the distilled water. The LB dipper raised the membrane at a rate of 5 mm/min, thinly coating solution onto the membrane. The modified electrodes and membranes were then tested on a hydrogen fuel cell. The control cell consisted of standard Pt electrodes with a loading of 0.1 mg/cm and an unmodified Nafion membrane that was soaked in distilled water. Sample electrodes/membranes were modified using Fe(CO)5-prGO, Fe (III)-prGO, Fe(III)AuPt-prGO and a setup using AuPt-prGO was made and tested to determine the effects of adding Fe as a catalyst. The maximum power output was determined by introducing purified H2 gas into the system at a flow rate of 80 ccm. To provide a uniform dispersion of the gas, 2.5 min were allowed to pass. The testing system calculated the ohms of Research Letter MRS COMMUNICATIONS • VOLUME 7 • ISSUE 2 • www.mrs.org/mrc ▪ 167 https://doi.org/10.1557/mrc.2017.14 Downloaded from https://www.cambridge.org/core. IP address: 54.191.40.80, on 21 Aug 2017 at 14:03:51, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. electrical resistance to automatically raise the current of the fuel cell by 0.05 amps every 30 s, beginning at 0 amps and continuing for a total of 7.5 min. Each setup was tested at least three times to activate electrodes until peak performance was exhibited. To test the CO resistance, the same process was then repeated using 0.1% (1000 ppm) of CO in the H2 gas feed. The control setup and the Fe(III)AuPt-prGO setup were each tested twice for CO resistance as this modified sample exhibited the highest power output.