Fuqiang Liu

Nafion/PTFE composite membranes for fuel cell applications

The composite membranes were prepared by impregnation of porous poly(tetrafluoroethylene) membranes with a 5 wt% Nafion solution. Scanning electron microscope micrographs of composite membranes show the surface and cross section of poly(tetrafluoroethylene) membranes were covered and filled with Nafion resin. Comparison of physical properties and fuel cell performance of composite membranes with those of Nafion membranes (DuPont Co) is presented. The composite membrane has better thermal stability and gas barrier property but worse ionic conductivity than Nafion membrane. Though the composite membrane has a lower conductivity than Nafion membrane, however, owing to the thinner thickness of composite membrane (in thickness of 20±5 µm) than Nafion-115 (in thickness of 125 µm) and Nafion-117 (in thickness of 175 µm) membranes, the composite membrane has a shorter H+ ion transporting pathway and thus a higher conductance (conductance = conductivity/membrane thickness) than Nafion-115 and Nafion-117 membranes. Thus the composite membrane has a better fuel cell performance than Nafion-117 and Nafion-115 membranes. In this report, we show that our composite membrane has a fuel cell performance similar to Nafion-112 membrane (in thickness of 50 µm).

Development of novel self-humidifying composite membranes for fuel cells

A novel preparation method for self-humidifying membranes of proton exchange membrane fuel cells (PEMFCs) was developed. Using solution-cast method, PTFE porous substrates in these composite membranes can increase their strength and distribute self-humidifying layers adjacent to the anode side. Compared with the cells fabricated with ordinary membranes, the performance of the cells with these self-humidifying proton exchange membranes (PEMs) are dramatically improved in both cell voltage and the current density under dry conditions, and the cell using the Pt/C-PEM shows the best and most stable performance. EIS technique revealed that these self-humidifying composite membranes could minimize membrane conductivity loss under dry conditions.

Low crossover of methanol and water through thin membranes in direct methanol fuel cells

Low crossover of both methanol and water through a polymer membrane in a direct methanol fuel cell (DMFC) is essential for using high concentration methanol in portable power applications. A novel design of the membrane-electrolyte assembly (MEA) has been developed in this work to attain low methanol crossover, low water crossover, and high cell performance simultaneously. The anode catalyst layer, in the form of a catalyzed diffusion medium (CDM), serves as a methanol diffusion barrier to reduce methanol crossover. In addition, a highly hydrophobic cathode microporous layer (MPL) is employed to build up the hydraulic pressure at the cathode and hence drive the product water from the cathode into the anode to offset the water dragged by electro-osmosis. The new MEA, consisting of a CDM anode, a thin Nafion membrane, and a carbon cloth precoated with an MPL on the cathode, is shown to attain: (i) a net water transport coefficient through the membrane smaller than 0.8 at 60°C and 0.4 at 50°C; (ii) fuel efficiency of ∼80%; and (iii) a steady-state power density of 60 mW/cm 2 at ca. 0.4V and 60°C with low stoichiometric flow rates of ambient dry air and 3 M methanol solution.

Degradation mechanism of polystyrene sulfonic acid membrane and application of its composite membranes in fuel cells

The lifetime behavior of a H2/O2 proton exchange membrane (PEM) fuel cell with polystyrene sulfonic acid (PSSA) membrane have been investigated in order to give an insight into the degradation mechanism of the PSSA membrane. The distribution of sulfur concentration in the cross section of the PSSA membrane was measured by energy dispersive analysis of X-ray, and the chemical composition of the PSSA membrane was characterized by infrared spectroscopy before and after the lifetime experiment. The degradation mechanism of the PSSA membrane is postulated as: the oxygen reduction at the cathode proceeds through some peroxide intermediates during the fuel cell operation, and these intermediates have strong oxidative ability and may chemically attack the tertiary hydrogen at the α-carbon of the PSSA; the degradation of the PSSA membrane mainly takes place at the cathode side of the cell, and the loss of the aromatic rings and the SO3− groups simultaneously occurs from the PSSA membrane. A new kind of the PSSA-Nafion composite membrane, where the Nafion membrane is bonded with the PSSA membrane and located at the cathode of the cell, was designed to prevent oxidation degradation of the PSSA membrane in fuel cells. The performances of fuel cells with PSSA-Nafion101 and PSSA-recast Nafion composite membranes are demonstrated to be stable after 835 h and 240 h, respectively.

Water Transport Through Nafion 112 Membrane in DMFCs

Water management has emerged as a significant challenge in portable direct methanol fuel cells, DMFCs. Excessive water crossover through the membrane causes water loss in the anode and flooding in the cathode. We report anovel DMFC design based on Nafion 112 and a cathode gas-diffusion layer coated with a microporous layer. This DMFC operated with ambient air yielded a stable power density of 56 mW/cm 2 at 60°C, while the net water transport coefficient was only 0.64 compared to a typical value of 3 for Nafion 117 membranes. In addition, the fuel efficiency was about 74-92%.

Mixed potential in a direct methanol fuel cell

A mathematical model for the cathode of a direct methanol fuel cell DMFC is developed to investigate two-phase transport in the catalyst layer CL and to elucidate the mechanism of cathode mixed potential due to oxidation of crossover methanol. A coupled model of two-phase species transport and multistep electrochemical kinetics, including simultaneous oxygen reduction, methanol oxidation, and gas phase chemical reaction, is presented. The model predictions agree favorably with experiments of cathode mixed potential, and the predicted profiles of water saturation, oxygen concentration, and overpotential along the CL thickness further reveal the profound interplay between multiple reactions and the transport of oxygen and water. It is shown that in the presence of methanol crossover, the DMFC cathode is operated at higher overpotential and water saturation, with larger oxygen transport loss than that in the H2/air counterpart. The model results also indicate that reducing the cathode CL thickness can facilitate both liquid water removal and oxygen transport through the CL, leading to improved cathode performance.

Synthesis of highly active and stable Au–PtCu core–shell nanoparticles for oxygen reduction reaction

Au-PtCu core-shell nanoparticles were successfully synthesized via galvanic replacement of Cu by Pt on hollow Au nano-spheres. Characterizations of the nanoparticles were conducted by X-ray diffraction (XRD), transmission electron microscopy (TEM), and electrochemical measurements. Results indicate 2-2.5 times higher specific activity and mass activity of the Au-PtCu catalysts than commercial Pt black and Pt/C in oxygen reduction reaction (ORR), measured in a rotating disk electrode system. Besides, thinner PtCu coating (25 nm thick, deposition time of 20 min) on the hollow Au nano-spheres demonstrated a pronounced CO oxidation peak shift (by 0.13 V) and long-term durability probably due to the unique core-shell structure and strong electronic coupling between the Au core and the PtCu shell.

Nonequilibrium Phase Transformation and Particle Shape Effect in LiFePO4 Materials for Li-Ion Batteries

The nonequilibrium phase transformation and particle shape effects in LiFePO4 materials of Li-ion batteries are explored in this work. A continuum model employing the mushy-zone (MZ) approach, accounting for sluggish Li diffusion across the two-phase boundary, has been developed to study the kinetically-induced nonequilibrium phenomenon in Li-ion batteries. A theoretical analysis is presented to show that the nonequilibrium miscibility gap expands and shifts to higher Li composition at high discharge rates, due to insufficient compositional readjustments at the two-phase boundary. Furthermore, critical effects of particle shape on nonequilibrium phase transformation and discharge capacity have been discovered by the model.

Pt-C/SPEEK/PTFE Self-Humidifying Composite Membrane for Fuel Cells

A Pt-C/sulfonated poly~ether ether ketone!/polytetrafluoroethylene ~Pt-C/SPEEK/PTFE! self-humidifying composite membrane for the H2 /O2 fuel cell was developed. Compared with the performance of the cells prepared with SPEEK/PTFE composite membranes, the performance with the self-humidifying composite membranes was appreciably improved when holding the humidity of cathode gas constant. The Pt-C/SPEEK/PTFE composite membrane can supply water to the anode electrode side

Au/Pd core-shell nanoparticles with varied hollow Au cores for enhanced formic acid oxidation.

A facile method has been developed to synthesize Au/Pd core-shell nanoparticles via galvanic replacement of Cu by Pd on hollow Au nanospheres. The unique nanoparticles were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, ultraviolet–visible spectroscopy, and electrochemical measurements. When the concentration of the Au solution was decreased, grain size of the polycrystalline hollow Au nanospheres was reduced, and the structures became highly porous. After the Pd shell formed on these Au nanospheres, the morphology and structure of the Au/Pd nanoparticles varied and hence significantly affected the catalytic properties. The Au/Pd nanoparticles synthesized with reduced Au concentrations showed higher formic acid oxidation activity (0.93 mA cm-2 at 0.3 V) than the commercial Pd black (0.85 mA cm-2 at 0.3 V), suggesting a promising candidate as fuel cell catalysts. In addition, the Au/Pd nanoparticles displayed lower CO-stripping potential, improved stability, and higher durability compared to the Pd black due to their unique core-shell structures tuned by Au core morphologies.