Thomas I. Valdez

Performance of Direct Methanol Fuel Cells with Sputter‐Deposited Anode Catalyst Layers

Performance of direct methanol fuel cells with sputter-deposited Pt-Ru anodes was investigated. The thin film catalyst layers w ere characterized using X-ray diffraction, energy dispersive X-ray analysis, Rutherford backscattering spectroscopy, and X-ray photoelectron spectroscopy. Different catalyst loadings and membrane electrode assembly (MEA) fabrication processes were tested. The maximum power density achieved at 90°C was 100 mW/cm 2, and almost 75 mW/cm 2 was attained with a loading of only 0.03 mg/cm2. The results demonstrate that a catalyst utilization of at least 2300 mW/mg can be achieved at current densities ranging from 260 to 380 mA/cm2. The application of the sputter-deposition method for MEA fabrication is particularly attractive for commercialization of direct methanol fuel cell technology.

Recent advances in PEM liquid-feed direct methanol fuel cells

A direct methanol-air fuel cell operating at near atmospheric pressure, low-flow rate air, and at temperatures close to 60/spl deg/C would tremendously enlarge the scope of potential applications. While earlier studies have reported performance with oxygen, the present study focuses on characterizing the performance of a PEM liquid feed direct methanol-air cell consisting of components developed in house. These cells employ Pt-Ru catalyst in the anode, Pt at the cathode and Nafion 117 as the PEM. The effect of pressure, flow rate of air and temperature on cell performance has been studied. With air, the performance level is as high as 0.437 V at 300 mA/cm/sup 2/ (90/spl deg/C, 20 psig, and excess air flow) has been attained. Even more significant is the performance level at 60/spl deg/C, 1 atm and low flow rates of air (3-5 times stoichiometric), which is 0.4 V at 150 mA/cm/sup 2/. Individual electrode potentials for the methanol and air electrode have been separated and analyzed. Fuel crossover rates and the impact of fuel crossover on the performance of the air electrode have also been measured. The study identifies issues specific to the methanol-air fuel cell and provides a basis for improvement strategies.

Design and Operation of an Electrochemical Methanol Concentration Sensor for Direct Methanol Fuel Cell Systems

The development of a 150-Watt packaged power source based on liquid feed direct methanol fuel cells is being pursued currently at the Jet propulsion Laboratory for defense applications. In our studies we find that the concentration of methanol in the fuel circulation loop affects the electrical performance and efficiency the direct methanol fuel cell systems significantly. The practical operation of direct methanol fuel cell systems, therefore, requires accurate monitoring and control of methanol concentration. The present paper reports on the principle and demonstration of an in-house developed electrochemical sensor suitable for direct methanol fuel cell systems.

Recent advances in direct methanol fuel cells

The direct methanol fuel cell is based on the electro-oxidation of an aqueous solution of methanol in a polymer electrolyte membrane fuel cell without the use of a fuel processor. The electro-oxidation of methanol occurs on platinum-ruthenium catalyst at the anode and the reduction of oxygen occurs on platinum catalyst at the cathode. After the initial concept development at the Jet Propulsion Laboratory (JPL), there has been considerable development of methanol fuel cell (DMFC) technology at the JPL and at various other institutions under programs sponsored by DOD and DOE. Significant improvements in power density, efficiency, and life have been demonstrated at the cell and stack level. These advances in the performance of direct methanol fuel cells are sufficiently attractive for the design of complete power systems. Portable power sources, in the range of 50-150 W, based on this technology are currently being considered for various military applications. The development of a 150 W direct methanol fuel cell power system is being pursued at the Jet Propulsion Laboratory (JPL) under DARPA funding. This paper summarizes some of the progress in the development of cells, stacks and systems.

Direct methanol fuel cell for portable applications

A five cell direct methanol fuel cell stack has been developed at the Jet Propulsion Laboratory. Currently, direct methanol fuel cell technology is being incorporated into a system for portable applications. Electrochemical performance and its dependence on flow rate and temperature for a five cell stack are presented. Water transport data, and water transport mechanisms for direct methanol fuel cells are discussed. Stack response to pulse loads has been characterized. Implications of stack performance and operating conditions on system design have been addressed.

Investigation of Ruthenium Dissolution in Advanced Membrane Electrode Assemblies for Direct Methanol Based Fuel Cell Stacks

The mechanism of ruthenium dissolution in Direct Methanol Fuel Cell (DMFC) Membrane-Electrode-Assemblies (MEAs) was investigated by preparing various MEAs and subjecting them to continuous operation for a period of 250 hours. All of the MEAs exhibited voltage decay. The voltage decay for MEAs operating at 40 o C, 0.5 M methanol was in the range of 6 to 48 mV at an applied current of 50-mA/cm 2 and 67 to 150 mV at an applied current of 100mA/cm 2 . The technique of anode polarization was used to determine the source of degradation and revealed that anode performance in these MEAs was constant and that the cathode was more sensitive to duration operation. EDAX analysis was performed on each cell to determine if ruthenium had dissolved from the anode and precipitated onto the cathode, no ruthenium was found on the MEA. This paper discusses stack properties that can promote ruthenium dissolution in DMFC MEAs.

Development of a 150-watt direct methanol fuel cell system

Stack development for a Nafion(R) based 150-watt direct methanol fuel cell (DMFC) system is discussed in this paper. Single cell data for a membrane electrode assembly (MEA) that can operate at low air stoichiometry is presented. The stack operating conditions for achieving a water balance have been determined to be 55/spl deg/C 0.5 M MeOH at a maximum of 1.75 times air stoichiometry at 100 mA/cm/sup 2/. Single cells with a 25-cm/sup 2/ active area have been operated in this regime and can maintain an average cell voltage of 0.43 V at 100 mA/cm/sup 2/ for 120 minutes with a cell voltage decay of 0.2 mV/min. A five-cell stack with a 80-cm/sup 2/ active area, scaled up from the single cell, was capable of sustaining 100-mA/cm/sup 2/ load at a 1.75 air stoichiometry for over 70 hours, with a voltage decay of the order of 2 mV/hr. Voltage decay is reversible by purging excess water in the cathode.

Direct methanol fuel cells-status, challenges and prospects

The status of direct methanol fuel cell technology with respect to power density, efficiency and integrated system operation have been summarized. The key challenge in improving power density is combining with operation at low air flow rates in order to maintain a water balance, and achieve attractive system mass and size. Improved catalysts and membranes with low methanol permeability are key to achieving these improvements. Challenges relating to miniature DMFC for battery replacement are discussed. Possibilities of reduction in catalyst and membrane cost suggest that premium power applications (100 W-5 kW) could be an early point of entry for DMFC into commercial markets.

Commercialization of a direct methanol fuel cell system

This paper describes a major breakthrough in energy technology developed at the Jet Propulsion Laboratory that can be used in a wide variety of portable, remote and transportation applications without polluting the environment. The status, performance, and design considerations of the JPL non-polluting, Direct Methanol, Fuel Cell system for consumer equipment and transportation applications are reported herein. This new fuel cell technology utilizes the direct oxidation of a 3% aqueous liquid methanol solution as the fuel and air (O2) as the oxidant. The only products are CO2 and water. Therefore, because recharging can be accomplished by refueling with methanol, vehicles can enjoy unlimited range and extended use compared to battery operated devices requiring recharge time and power accessibility.

Implementation of energy harvesting system for powering thermal gliders for long duration ocean research

The exploration of the Earths oceans is aided by autonomous underwater vehicles (AUVs). AUVs in use today include floats and gliders; they can be deployed to profile salinity, temperature and pressure of the ocean at depths of up to 2 km. Both the floats and gliders typically control buoyancy by filling and deflating an external bladder with a hydraulic fluid delivered by an electrical pump. The operation time of an AUV is limited by energy storage. For floats, such as the Argo float, the operating duration is approximately 5 years with the capability to dive once every 10 days. For electric gliders, such as the deep G2 Slocum, the mission duration can be up to one year with lithium primary batteries. An energy storage system has been developed that can harvest energy from the temperature differences at various depths of the ocean. This system was demonstrated on an Argo style float and has been implemented in a thermal version of the Slocum glider. The energy harvesting system is based on a phase change material with a freeze thaw cycle that pressurizes hydraulic oil that is converted to electrical energy. The thermal Slocum glider does not use an electrical pump, but harvested thermal energy to control buoyancy. The goal for the thermal Slocum glider is for persistent ocean operation for a duration of up to 10 years. A thermal powered glider with an energy harvesting system as described can collect conductivity, temperature, and pressure data and deliver it to the National Data Buoy Center (NDBC) Glider Data Monitoring System and the World Meteorological Organization (WMO) Global Telecommunications System (GTS). Feeding into operational modeling centers such as the National Centers for Environmental Prediction (NCEP) and the U.S. Naval Observatory (NAVO), this data will enable advanced climate predictions over a timespan not currently achievable with present technology. Current testing of the thermal powered Slocum glider is to determine the durability of the technology and quantify the glider system design. Previous issues with this technology included energy storage system management and glider mechanical limitations. Our objective is to learn how to fly an energy harvesting thermal glider that interacts with the ocean environment efficiently. We would also like to establish the latitudinal range of operation. This thermal powered Slocumglider, dubbed Clark, after the famous explorer duo Lewis and Clark, has been deployed off of St. Thomas for flight dynamics and durability testing. The following paper will discuss the deployment and testing of the thermal powered Slocum glider. We will also discuss the advantages of ocean energy harvesting technology for oceanographic research.