Ex situ investigation of the proton exchange membrane chemical decomposition

The durability of Nafion^® polymer electrolyte membranes (PEMs) with potential application in PEM fuel cells has been investigated using accelerated durability tests to understand their degradation mechanism. After the attack by Fenton radicals, the Nafion^®111 membranes and the solution produced were collected for analysis. The existence of F ions in the solution indicated the chemical decomposition of the Nafion^® membranes during radical attacks. The F^- emission rate (FER) was about , corresponding to 0.024 wt% of F released from the membrane per hour. The NMR and FTIR spectrums demonstrated the polymer fragments mostly existed as whole side chains of the Nafion^® membrane. This result revealed that the degradation was originated from the decomposition of polymer main chain. Furthermore, the reflectance-FTIR revealed that the degradation of the PEMs was from the decomposition of the repeating units in the polymer main chains. With the increased loss of repeating units, small bubbles with the diameter of several microns started to form in Nafion^® membrane. These bubbles made the membrane vulnerable to hazards of gas crossover, which further led a catastrophic failure of the proton exchange membrane.     

A study on fabrication of sulfonated poly(ether ether ketone)-based membrane-electrode assemblies for polymer electrolyte membrane fuel cells

The porosity effect of catalyst electrodes in membrane-electrode assemblies (MEAs) using a hydrocarbon-based polymer as electrolyte and ionomer was investigated on physical and electrochemical properties by varying the content of ionomer binder (dry condition) in the catalyst electrodes. The MEAs were compared with the Nafion^®-based MEA using Nafion^® 112 and 5 wt.% ionomer solution (EW = 1100) in terms of porosity values, scanning electron microscopic images, Nyquist plots, dielectric spectra and I–V polarization curves. In this study, sulfonated poly(ether ether ketone) (SPEEK) membranes with 25 ± 5 μm of thickness and 5 wt.% ionomer solutions have been prepared. The prepared membranes were characterized in terms of FT-IR, DSC and proton conductivity. Proton conductivity of the SPEEK membranes was compared with one of the Nafion^® membranes with relative humidity. The porosity of the catalyst electrodes was calculated using the properties of catalyst, ionomer solution and solvent. As a result, the performance of the new type polymer (i.e., SPEEK in this study)-based MEA with the similar membrane conductivity and porosity of the catalyst electrode in the Nafion^® MEA was similar to that of the Nafion^® MEA.     

Improvement of cathode–electrolyte interfaces of tubular solid oxide fuel cells by fabricating dense YSZ electrolyte membranes with indented surfaces

To improve cathode–electrolyte interfaces of solid oxide fuel cells (SOFCs), dense YSZ electrolyte membranes with indented surfaces were fabricated on tubular NiO/YSZ anode supports by two comparable methods. Electrochemistry impedance spectroscopy (EIS) and current–voltage tests of the cells were carried out to characterize the cathode–electrolyte interfaces. Results showed that the electrode polarization resistances of the modified cells were reduced by 52% and 35% at 700 °C, and the maximum power densities of cells were remarkably increased, even by 146.6% and 117.8% at lower temperature (700 °C), respectively. The indented surfaces extended the active zone of cathode and enhanced interfacial adhesion, which led to the major improvement in the cell performance.     

Mechanical properties of a reinforced composite polymer electrolyte membrane and its simulated performance in PEM fuel cells

The hygro-thermo-mechanical properties and response of a class of reinforced perfluorosulfonic acid membranes (PFSA), that has potential application as an electrolyte in polymer fuel cells, are investigated through both experimental and numerical modeling means. A critical set of material properties, including Young's modulus, proportional limit stress, break stress and break strain, is determined for a range of temperature and humidity levels in a custom-built environmental test apparatus. The swelling strains are also determined as functions of temperature and humidity level. To elucidate the mechanical response and the potential effect these properties have on the mechanical durability, mechanics-based simulations are performed using the finite element method (ABAQUS). The results indicate that the relatively high strength of the experimental membrane, in combination with its relatively low in-plane swelling due to water absorption, should have a positive influence on membrane durability, potentially leading to longer life times for polymer electrolyte membrane fuel cells (PEMFC).     

Solid oxide fuel cells with bi-layered electrolyte structure

In this work, we have developed solid oxide fuel cells with a bi-layered electrolyte of 2 μm SSZ and 4 μm SDC using tape casting, screen printing, and co-firing processes. The cell reached power densities of 0.54 W cm⁻² at 650 °C and 0.85 W cm⁻² at 700 °C, with open circuit voltage (OCV) values larger than 1.02 V. The electrical leaking between anode and cathode through an SDC electrolyte has been blocked in the bi-layered electrolyte structure. However, both the electrolyte resistance (R_el) and electrode polarization resistance (R_p,a+c) increased in comparison to cells with single-layered SDC electrolytes. The formation of a solid solution of (Ce, Zr)O_2−x during sintering process and the flaws in the bi-layered electrolyte structure seem to be the main causes for the increase in the R_el value (0.32 Ω cm²) at 650 °C, which is almost one order of magnitude higher than the calculated value.     

Macroporous nanocomposite polymer electrolyte for lithium-ion batteries

A novel macroporous nanocomposite polymer membrane (NCPM) based on poly(vinylidene difluoride-co-hexafluoropropylene) [P(VDF-HFP)] copolymer was prepared by in situ hydrolysis of Ti(OC₄H₉)₄ using a non-solvent-induced phase separation technique. SEM micrograph shows that the yielding TiO₂ nanoparticles are dispersed uniformly in the polymer matrix and there are a lot of spherical macropores connecting with each other by some smaller pores. DSC results exhibit that the crystallinity of polymer matrix decreases with the incorporation of TiO₂ nanoparticles. The tensile stress of the NCPM is 9.69 MPa and its fracture strain 74.4%. After immersion in 1.0 mol l⁻¹ LiPF₆/ethyl carbonate (EC)–dimethyl carbonate (DMC), the ionic conductivity of the obtained nanocomposite polymer electrolyte (NCPE) is 0.98 × 10⁻³ S cm⁻¹ at 20 °C. Lithium-ion batteries, which use this kind of NCPE as the separator and electrolyte, display good discharging performance at different current densities, presenting promise for its practical application.     

An investigation into nickel implanted 316L stainless steel as a bipolar plate for PEM fuel cell

A nickel-rich layer about 100 μm in thickness with improved conductivity was formed on the surface of austenitic stainless steel 316L (SS316L) by ion implantation. The effect of ion implantation on the corrosion behavior of SS316L was investigated in 0.5 M H₂SO₄ with 2 ppm HF solution at 80 °C by potentiodynamic test. In order to investigate the chemical stability of the ion implanted SS316L, the potentiostatic test was conducted in an accelerated cathode environment and the solutions after the potentiostatic test were analyzed by inductively coupled plasma atomic emission spectrometer (ICP-AES). The results of potentiodynamic test show that the corrosion potential of SS316L is shifted toward the positive direction from −0.3 V versus SCE to −0.05 V versus SCE in anode environment and the passivation current density at 0.6 V is reduced from 11.26 to 7.00 μA cm⁻² in the cathode environment with an ion implantation dose of 3 × 10¹⁷ ions cm⁻². The potentiostatic test results indicate that the nickel implanted SS316L has higher chemical stability in the accelerated cathode environment than the bare SS316L, due to the increased amount of metallic Ni in the passive layer. The ICP results are in agreement with the electrochemical test results that the bare SS316L has the highest dissolution rate in both cathode and anode environments and the Ni implantation markedly reduces the dissolution rate. A significant improvement of interfacial contact resistance (ICR) is achieved for the SS316L implanted with nickel as compared to the bare SS316L, which is attributed to the reduction in passive layer thickness caused by the nickel implantation. The ICR values for implanted specimens increase with increasing dose.     

Development of tellurium-modified carbon catalysts for oxygen reduction reaction in PEM fuel cells

Tellurium (Te)-modified carbon catalyst for oxygen reduction reaction was prepared through chemical reduction of telluric acid followed by the pyrolysis process at elevated temperatures. The catalyst was found to be active for oxygen reduction reaction. High-temperature pyrolysis plays a crucial role in the formation of the active sites of the catalysts. When the pyrolysis was conducted at 1000 °C, the catalyst exhibited the onset potential for oxygen reduction as high as 0.78 V vs. NHE and generated less than 1% H₂O₂ during oxygen reduction. The performance of the membrane–electrode assembly prepared with the Te-modified carbon catalyst was also evaluated.     

All-solid-state rechargeable lithium batteries with Li2S as a positive electrode material

The Li₂S–Cu composite electrode materials were prepared by mechanical milling and applied to all-solid-state lithium cells using the Li₂S–P₂S₅ glass–ceramic electrolyte. The addition of Cu and the mechanical activation improved the electrochemical performance of Li₂S in all-solid-state cells. The In/Li₂S–Cu cells were charged and then discharged at room temperature, suggesting that Li₂S was utilized as a lithium source. The cells exhibited high discharge capacity of about 490 mAh g⁻¹ at the 1st cycle. The SEM and EDX analyses suggested that the amorphous Li_xCuS domain was partially formed by milling, and the domain played an important role in the improvement of capacity. The electrochemical reaction mechanism of the Li₂S–Cu composites was discussed on the basis of the mechanism of the S–Cu composite electrode.