Rongzhong Jiang

Comparative studies of methanol crossover and cell performance for a DMFC

Fuel (methanol) crossover through the polymeric electrolyte membrane in a single direct methanol fuel cell (DMFC) was determined by monitoring the amount of CO 2 produced from methanol oxidation. Instead of measuring CO 2 from only the cathode by a conventional method, the amounts of CO 2 at both of the cathode and the anode were determined in the present study. Gravimetric determination of BaCO 3 was employed to accurately analyze the amount of CO 2 . The equivalent current of methanol crossover can be calculated from the discharge current of the fuel cell and the sum of dry BaCO 3 precipitate collected at the anode and the cathode exhausts. The common experimental deviation of measuring methanol crossover caused by CO 2 permeation through polymeric electrolyte membrane can be corrected with the proposed method. These data of methanol crossover were compared with the data of single cell polarization behaviors at different methanol concentrations and different temperatures. The energy density of the DMFC is not only dependent on the cell discharge performance but also significantly dependent on the faradaic efficiency that is directly linked to methanol crossover. Under the optimized operating conditions, 1.0 M methanol at 60°C, the DMFC has an energy density of 1800 Wh/kg based on pure methanol.

Fuel Crossover and Energy Conversion in Lifetime Operation of Direct Methanol Fuel Cells

A method of simultaneous measurement of multiple electrochemical parameters was employed to examine the lifetime operation of direct methanol fuel cells (DMFCs), which includes not only time duration, operational voltage and current, but also the fuel crossover, energy density, cumulative capacity, and cumulative energy. The lifetime performance of two types of DMFCs under different operating conditions was quantitatively compared based on their maximum capability to generate electric energy. A phenomenon of delamination between the anode electrode and electrolyte membrane was observed during lifetime operation. The long-term effects of heat, methanol solution, and CO 2 gas may gradually reduce the binding force between the anode electrode and the electrolyte membrane until its physical separation, which results in gradual increase of the resistance of the interface and causes discharge performance degradation. Such performance degradation from the anode side is apparently different from that of ruthenium crossover, which was previously reported, that inhibits the catalytic reduction of oxygen by the presence of ruthenium on the cathode catalyst.

Water Crossover: A Challenge to DMFC System I. Experimental Determination of Water Crossover

Water and fuel crossover in a direct methanol fuel cell (DMFC) stack under various operating conditions were quantitatively determined by a method of mass balance analysis of fuel and water. With increasing the discharge voltage, the fuel crossover rate increases, but the water crossover decreases slightly. With increasing the operating temperature, both the water and fuel crossover rates increase significantly. The amount of water permeated across the electrolyte membrane is as much as eight times that of water produced by methanol electrochemical oxidation at the anode. There are three types of water crossover, including proton osmotic drag, methanol migration drag, and water spontaneous crossover. For a 24-cell DMFC stack that uses 1.0 M methanol and operates at 62°C under 8 V discharge, the water crossover rates for proton osmotic drag, methanol migration drag, and water spontaneous crossover are 0.661, 0.243, and 1.186 mg min -1 cell -1 cm -2 , respectively. Among them, water spontaneous crossover is the most significant. There is a significant portion of water evaporation at the cathode outlet which is highly dependent on the operating conditions and relative humidity. Water crossover and evaporation result in great challenges for a DMFC system to recycle the cathode water.

Nano-Structured Bio-Inorganic Hybrid Material for High Performing Oxygen Reduction Catalyst

In this study, we demonstrate a non-Pt nanostructured bioinorganic hybrid (BIH) catalyst for catalytic oxygen reduction in alkaline media. This catalyst was synthesized through biomaterial hemin, nanostructured Ag-Co alloy, and graphene nano platelets (GNP) by heat-treatment and ultrasonically processing. This hybrid catalyst has the advantages of the combined features of these bio and inorganic materials. A 10-fold improvement in catalytic activity (at 0.8 V vs RHE) is achieved in comparison of pure Ag nanoparticles (20-40 nm). The hybrid catalyst reaches 80% activity (at 0.8 V vs RHE) of the state-of-the-art catalyst (containing 40% Pt and 60% active carbon). Comparable catalytic stability for the hybrid catalyst with the Pt catalyst is observed by chronoamperometric experiment. The hybrid catalyst catalyzes 4-electron oxygen reduction to produce water with fast kinetic rate. The rate constant obtained from the hybrid catalyst (at 0.6 V vs RHE) is 4 times higher than that of pure Ag/GNP catalyst. A catalytic model is proposed to explain the oxygen reduction reaction at the BIH catalyst.

Water Crossover: A Challenge to DMFC System II. Simulation of Water Recycling in a 20 W DMFC System

Variations of methanol concentration and solution volume in an anode water tank with operating time in a 20 W direct methanol fuel cell (DMFC) system that uses pure methanol and recycles the cathode water were simulated. The basic parameters for simulation of the DMFC system were acquired from a model DMFC stack. Many factors in a DMFC system design, such as the average cell voltage, operating temperature, water recycling percentage, and initial solution volume, affect water consumption, generation, and evaporation significantly, and in turn, cause gradual changes of methanol concentration and solution volume. The water generation rate varies significantly with operating temperature, but only slightly with average cell voltage. At 62 and 42°C, the generation rates of recyclable water are 1.44 and 0.88 g/min, respectively, for a DMFC system using 1.0 M methanol in an anode water tank and operating at 8.0 V (or average cell voltage 0.33 V). To balance water consumption and generation, it is necessary to recycle 98% water at an average cell voltage of 0.42 V, and to recycle 90% water at an average cell voltage of 0.33 V for the DMFC system operating at 62°C. Only 80% water needs to be recycled for the same DMFC system at an operating temperature of 42°C and average cell voltage of 0.33 V.

Non-precious Mn1.5Co1.5O4–FeNx/C nanocomposite as a synergistic catalyst for oxygen reduction in alkaline media

In this study we show a method of preparing a high performing catalyst by designing functional nano boundaries in a nanocomposite material. A non-precious nanocomposite material composed of spinel Mn1.5Co1.5O4 nano crystals and FeNx-functioned graphene nano platelets (FeNx/C) was synthesized by an ultrasonic process. The crystal structure and elemental composition of the bimetal oxide were determined by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The surface morphology of the Mn1.5Co1.5O4–FeNx/C nanocomposite was characterized with transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The catalytic activity for the oxygen reduction reaction (ORR) was analyzed by an electrochemical method. The enhancement of activity for the ORR at the nanocomposite material is attributed to double synergistic effects from the bimetal particles and the FeNx/C nano sheets. The nanocomposite material is able to catalyze 4-electron oxygen reduction to generate water in alkaline media with a high kinetic rate constant (7.6 × 10−2 cm s−1 at 0.7 V vs. reversible hydrogen electrode, RHE). Finally, the activity and stability of the nanocomposite material were compared with that of 40% Pt supported on active carbon (40% Pt/C), which reaches 95% activity and a comparable stability of 40% Pt/C at 0.7 V (vs. RHE).

Fuel cell powered small unmanned aerial systems (UASs) for extended endurance flights

Small unmanned aerial systems (UASs) have been used for military applications and have additional potential for commercial applications [1-4]. For the military, these systems provide valuable intelligence, surveillance, reconnaissance and target acquisition (ISRTA) capabilities for units at the infantry, battalion, and company levels. The small UASs are light-weight, manportable, can be hand-launched, and are capable of carrying payloads. Currently, most small UASs are powered by lithium-ion or lithium polymer batteries; however, the flight endurance is usually limited less than two hours and requires frequent battery replacement. Long endurance small UAS flights have been demonstrated through the implementation of a fuel cell system. For instance, a propane fueled solid oxide fuel cell (SOFC) stack has been used to power a small UAS and shown to extend mission flight time. The research and development efforts presented here not only apply to small UASs, but also provide merit to the viability of extending mission operations for other unmanned systems applications.